We have been playing Ars magica for over a year now.
Players have started to badger me in to convert to 5th edition. But why?
Can anyone give some good reasons for this?
All the cool kids are doing it! 
Sit vis vobiscum,
ShopKeepJon
To keep Atlas Games alive. They make great games! 
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ShopKeepJon
Parma magica works
Confidence works
Minor magical focus will make your magi character's numbers much more
interesting then they were in fourth
Vis study is balanced rather than clearly the best way to go
Study totals have been brought under control
The spell guidelines are more clear and more balanced
The laboratory chapter is more balanced and more clear
Combat is lots better (but not good, If you've house ruled the combat in fourth until it works I can't guarantee that fifth will be better)
Downtime actions are clear for all characters not just magi
Pretty much every individual mechanic has been cleaned up. Creating a new character is more of a pain but other than that, you should see everything run faster and smoother with less reference to books.
OK, on a more serious note...
Everything in 5th Edition is just a little better explained; the Realms, Art and Form guidelines, the structure of the Order, etc. This, among other things, makes running the game easier and casting spontaneous magic much simpler to use and adjudicate.
Hmm...now that I'm trying to explain it, I'm having a hard time...
The main thing is that in earlier editions we had to work fairly hard to make everything fit together, to feel right as a whole. 5th Edition just does. (Or, at least, much better than before.)
Maybe someone else can do a better job... 
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ShopKeepJon
That's what I wanted!
I just didn't type fast enough... 
Sit vis vobiscum,
ShopKeepJon
It has pros and cons of course.
Good points:
Parma magica is really cool and the power level of the characters is limited. The handling of the health levels. Story hook flaws. Very coherent. Better Hermetic background. Rules to create older characters.
Things to improve:
Focuses and the too much talent granting virtues make the characters overspecialized.
It supports only learning from books.
Some hints how the rules work in the practice and what is reasonable would help new and sometimes advanced players.
What the other people said! And on top of that, that the editor and authors have done a bloody good job on keeping the various supplements both flowing steadily and keeping them very much in sync with each other and the line.
A contrary view:
If you want to continue to embrace the world created by 4th Ed. et al, do not TOUCH the 5th Ed. It turns the logic of the milieu inside out when it comes to non-Hermetic traditions, makes magic cumulatively toxic, and has broken the balance of powers in the Realms, with the Dominion coming close to crushing anything and everything else.
There may be things worthwhile in the sourcebooks, but the psychic dissonance generated by the main book can be most unpleasant.
Indeed. But if that doesn't convince you, here...
Our group were playing the same campaign in Wales when the 5th edition came out.
We decided to wait until we've read and review the new edition before deciding if we were to convert or not.
When I got my book, it took me a few hours of reading to know we would convert. In my opinion, the 5th edition is better as the 4th.
It's as simple as that.
Eric Tyrrell up here explained the best objective reasons to convert, I think they should convince any "serious" player of Ars Mag.
For me, the stronger points are:
To sum all this up: the rules are better.
For me, the rules are almost all there is to gain from a published RPG system. The campaign background proper needs to be built using any material, from Ars Mag 5th, 4th, 3th, etc, internet, books, etc.
I can't agree with that because we kept the same "world" we used in our campaign when we converted from 4th to 5th. For us, it was easier to convert and "houserule" a few minor points, then to stay with 4th. 4th had too much problems in it compared to 5th. It goes as far as that !
I personally did not have experience the psychic dissonance that Arawan did.
The effects of magic being cumulatively toxic refers to, I suspect, the description of the effects of the gift in fifth which are more in detail then previous editions and no different than I had interpreted them in the past. However they aren't the way that everyone has interpreted them in the past because they usd to be ill-defined. Lots of folks seem to read them more direly than I did, than David claims is intended, and than is justified in my opinion from a sober unbiased reading of the text.
In any case the description of the gift may not be what you like.
"It turns the logic of the milieu inside out when it comes to non-Hermetic traditions" I believe that this refers to the fact that in fifth edition non-hermetics do not have parma equivalents. They have to use piece- meal defenses. That's it.
The thought behind this is that in the history of the order the parma was made out to be the breakthrough that was so important that folks wanted to join the order. Only in fourth ed.everybody else already had a similar trick. Hermetics can do Parma, other traditions can do other things.
"and has broken the balance of powers in the Realms, with the Dominion coming close to crushing anything and everything else."
Every edition of the game has claimed that the powers of the divine are infinite. Every edition of the game has said that magi are severely hampered in the dominion. Every edition of the game before fifth has failed to hamper magi in the dominion to any reasonable extent. In fifth the penalty for casting in the dominion is 3x's aura. The rules finally match the fluff.
So if you have the same reaction as Arwan, you'll want to:
- ignore a page of fluff about the effects of the gift.
- use the fourth edition exotic traditions just like you've been doing and
- use the numbers from the fourth ed. realm interaction chart.
In each of these cases we have the game rules lining up with the game fluff after four editions of cognitive dissonance. Yet if the setting implied by the rules rather than the texts of previous editions is more to your liking (and I don't mean to imply that it shouldn't be), you'll still (In my opinion) be better off with a few small changes to fifth than with the core rules of fourth.
Good question, many good answers.
Having only played 4th and 5th ed myself, I still have full knowledge of 2nd and somewhat of 3rd.
5th ed is IMHO by far the best edition!
Why you ask?
Spellcasting is better, with new Penetration rules and Spell Mastery
The Arts are better defined, especialy some of the confusion between Muto and Rego.
Spell Guidelines are more easily grasped and better coordinated
Lab rules are more streamlined, as all activities use the same basic mechanics. Also a good deal of fine expansions of certain activities/projects. Especially Familiars and Talismans have become more approachable and useful.
The system takes some getting use to regarding advancement. But with Abilities and Arts using the same scale, but abilities needing more points to raise a level, it's easier to differentiate in finer detail about an Ability. In 4th ed, 1 exp in Magic Theory was ok, and you could get no less from exposure while working in the lab. In 5th, you need 5 exp for the same effect, but you could differentiate better.
Personally I don't see what the troubles are with the description of The Gift. I think this gives a better description of it that I've ever read.
The additional materials are also very good. I like Covenants, and all the Houses of Hermes books.
There is one major reason to upgrade to 5th edition: They ain't making any more 4th edition material.
Other pros and cons balance out. Mechanically it is improved, but not as magnificently as some may suggest. ArM5 attracts rabid fans that insist that they have finally found the One True Edition at last. They haven't. 4th edition was a great game, and that's the reason many of us are here today. We already liked the game.
Yes, 5th edition is a definate improvement. Not 100% of everything was improved for the better, and I do agree with Arwan concerning artificial restrictions on Virtues and Flaws. But the core magic rules are much beter and there are a lot of cool bells and whistles. 5th edition Spell Mastery rocks.
However, what I am dissapointed about, and what I will continue to complain about always and forever, 5th edition has been casual and callous towards past cannon and storyline. As I said, I already liked the game. I did not appreciate revisions that disregarded the feelings past fans and pretty much trashed the history of my past sagas. I am still very much upset by that, and this is the single biggest criticism I have of 5th edition, and I will hold a grudge about this until they change things back the proper way.
But, having said that, the game still manages to hold my interest and provide me enjoyment, despite my criticisms. I suppose that must be a testament as to how good the material really is.
Methinks he's talking about Warping for non-magi, which quantifies something that was not so explicit before (just have a look at the 4th ed MuCo guidelines, which do mention long-term effects for transformation spells, or the botch rules for Gruagachan and Trollsynirs).
Methinks that 5th edition is not the first nor the last to attract rabid fans... and that that notion is most often found in those not in agreement with said preference... After all I do sincerly prefer blue over green - all in the eye of the beholder! Discussing preferences, especially heated, seems rather pointless at times.
I personally prefer 5th edition, even if I only took my troupe to it step by step and taking my time embracing it, but with every new supplements I'm assured in that preference.
Preference aside, I do think it's fair to say that 5th edition rules have been made more consistently across the supplements than before and they contain less discrepancies between fluff and crunch than before (whether one likes the current fluff/crunch or not).
Fiinally, I like others have mentioned, did not see the crunch change to either the effects of the Gift or the Dominion as very significant - as the current crunch fits the fluff we already adhered to in our troupe.
I'd just add my voice to the chorus in saying I think ArM5 is the best edition yet (of ArM3 and ArM4; didn't know Ars before).
ArM5's greatest advantage is in its consolidation and clarification of rules and setting under a more unified framework. It really things like things (from the relations of the Realms to how laboratory rules should work) have been thought out in advance, so everything fits together much more smoothly than in 4th edition.
I don't like all of ArM5. I think the edition, by now, is bloated with supplements that (by the nature of things) escalate the power level and create a more patchwork and rules-heavy game. I think curving the power level of magi wasn't done well enough, and that the rules for generating old characters still fall short of the power level equally-aged magi will reach, and that the power level of magi was not sufficiently weakened and is inconsistent with the setting at large. I don't like certain aspects of the combat rules (especially group fighting, and the lacks of tactics), of how Might is handled (a dragon's breath shouldn't expand Daily powers), of how certamen is handled (I still find it boring), and more.
But these all pale in comparison with the number of things ArM5 does right. The far more robust magic guidelines, with their clear delineation of Rego vs. Muto; the wonderful Sympathetic Magic rules, giving power to birth-dates, names, images, and so on; the fluid combat rules, making positioning and miniatures irrelevant (unlike THAT game); the consistent Laboratory rules, with their great freedom; the improved arcane experimentation table; the better, clearer and more flavorful, presentation of the rules and setting; the Story Flaws; Spell Mastery; the improved Magic Resistance; and more.
Specific examples? There are aspects of the 5th edition background that have been introduced on an ongoing basis and that contradict things I've introduced in my sagas, but I don't really care. I just ignore them. 
For instance, long before HoH: Societates came out with its info on the lineage of Pralix, I sort of inferrered from what it said in HoH: True Lineages that Pralix and her filii would practice magic derived from the necromantic traditions of Tremere and Tytalus. It's quite an important part of my saga background, actually, so I don't use the Line of Pralix as they're written up in Societates.
The thing is, a lot of the background given in previous editions was pretty vague and there were a lot of gaps for individual storyguides to fill in. I'd say that if your ideas from two years ago are contradicted by something that it says in a sourcebook from last year, the established background in your saga takes precedence.
House Flambeau.
I gotta calm down about this. I have been able to play my character the way I want and introduce bits of old Flambeau ore all the time and no one seems to notice any contradiction. And the crunch of the Flambeau chapter is so magnificent that it well makes up for the dissappointing revision.
To sum up my oppinions:
You should only change enditions if you want to. I wanted to, and I have no regrets. However, I'm known for wantonly ignoring any fluff which might not fit into what I've already defined. Since I play in a saga, which is a spin-off of an old 4th ed saga, which again is a spin-off of an even older 4th ed saga...
But with the changes in the defitions and fluff of various houses, nothing has been upset in my saga(s). And I even play a Flambeau Hoplite. It changes nothing for me, that Flambeau the Founder lived and died in a different way. I think we already weant above and beyond the stereotypical fire-maniacs back in the old days.
If one has more fantasy than a shovel he can resolve story changes between editions.
E.g. we play in the Transylvanian tribunal where it would be boring to play in the shadow of a Tremere domination. They are the majority still but detached into 2 factions based certain Hermetic law issues and inner struggle for power.