I've always thought the rules for Theurgy in Mystery Cults were incredibly flavorful and awesome. True Names, Synthemata, summoning daimons, each individual part of that chapter seemed like a super cool ability. How do you put it all together though? How do you create an effective character based around theurgy?
I like Theurgy also. It's flavorful and reflects actual magical tradition better than does standard AM Hermetic Magic. The problem is, from a game perspective, that a character really doesn't get much benefit out of the virtues, at least compared to the difficulty of initiating the lot of them.
I use Theurgy for magical traditions outside of the Order of Hermes. My preference is that Hermetic Magic have roots back in the Hermeticism of antiquity and not be only the legacy of Bonisagus. I've sketched up a Byzantine order of magicians who use Neoplatonist theurgy to cast spells only using the Theurgy virtues from Mysteries, with no access to standard formulaic or spontaneous magic. I also picture Zoroastrian magi as also using theurgy, with the added ability to work with Divine beings in this manner, although I haven't quite figured out how it would function in detail.
The only way (that I can see) that theurgy could be effective is if your GM was to make a lot of daimons who were worth their summoning vis. Nonhermetic powers, combat utility; if the daimons are good then theurgy is good.
Heck, how do you not? All the stuff is cool, and interlocked to the point that learning one mystery points to the next. Muto Viming your spells on the spot! Spells that are really spirits with minds, that can be told when to act, and who to act on! The real question is "How do the other players keep up?".
have ou actually used this, or did you just look at the descriptions?
Personally I've found it somewhat fidley and probably not worth it without relevant Potent Magic/Magical Focus.
From my experience, in terms of raw "oomph", theurgy is not really worth it ... unless you combine it with some very specific packages of virtues and flaws, in which case it's ok, but certainly not exceptionally powerful. I think this is the correct way to "tune" mystery magics: generally not worth the price, except for very specific niches for which they are slightly better than vanilla Hermetic magic.
Well, styles of play I guess. It's very easy to get "oomph" in Ars Magica, so look as you don't try to get "oomph" with everything. So I like "fidley". And if you want to combine the two, the ability to have an Archmage as your Familiar (p. 82, the Mysteries)! Book, Teacher, Powerful Ally and incredibly useful tool all in one! A possible +5 to understand Twilight! And an easy "in" to House Criamon if you want to go that way. And that's a lesser Theurgy virtue......
If your magus is going to do a lot of dealing with spirits and related, then theurgy is well worth it. The penetration bonuses from names of power (in addition to synthemata) can be very helpful. Get your magic theory and magic lore scores above 6 and away you go. If your magus is more interested in other areas of endeavor, then of course it would not be worth it. Having the mystery tuned such that it is definitly worth it for those who are interested and only of incidental benifit otherwise makes a lot of sense ot me.