The flaw has it's own rules and stuff but I feel like it could be a lot of fun to put more in depth roleplay into magic addiction.
My idea is to have the character repeatedly cast a free spontaneous spell out of habit. (keeps making his finger flash for a second or somthing like that) Making him obviously too willing to use magic for anything. (levitatng an apple rather than reaching for it ect.)
Has any one played a magic addicted character or though about how to RP one in a fun way?
If I would play a magic addicted magus, I would think about my nicotine addiction. So, all that you mentioned plus, say, pointlessly juggling this apple in the air during travel (making magus an obvious target for those hiding in the bushes), changing a colour of rags on the floor of tavern room a couple of times before falling asleep (and making everyone with Magic Sensitivity in the area), things like that. Every addiction can become more severe - tendency to engage in more strenous lab routines may develope (who needs off-time when you can feel magic 16 hours a day, 7 days a week!), addicted magus can even spend a season or two developing vim spells that serve only one purpose - to engulf caster in magic power (and cause warping, but who cares?). The next stage would be stealing vis and wasting from covenant stores (magic feels better if boosted, and no-one will notice that couple of pawns are missing, yeah?) and so on...
Every addiction is serious problem. Addicted magi led worse lives and meet Final Twilight earlier.
As with all addictions , choose a theme , any personality flaws would provide colour.
The Meddler Flaw (page 56 ArM05) would give ample excuses to spellcast.
My choice would probably be Rego Craft spells.
The peasants look poor and wretched .
Certainly my "Scour the Unwashed Multitude" spell will be appreciated.
(Cast at night , while they sleep ; to awaken to a fresh , new day)
How drab the area is without beautiful roses wafting their sublime species.
That "Rapid Growth of the Floral Bounty" is just to hand.
(Cast over a season , so unnatural growth does not "interfere with mundanes")
Simply choose an area of life with people (or their livestock)
and you can design spells to cast repeatedly.
You could bother taking an affinity with Finesse ,
but it reduces the repeat options if your craft spells
always work well the first time.
and here I thought you meant that infernal card game thingie
I've got a Magic Addicted Tremere I'm playing right now, and I take a different tack. Rather than the "constant-fix" style addiction, I think the rules lend themselves more to a "binge" style of addiction. He avoids ever casting spells, because when he does, he knows he'll find it hard to stop... he'll just keep casting over and over until he passes out, waking up groggy with a "what did I do last night" perspective. 
This is pretty much true i think and even though the roll to resist is easy enough that you pretty much should never pass out from you addiction.
laying an addict like a recovering alchoholic would be prolly as much fun as playing one like a coke fiend.
You keep promising your wife that you'll never cast again and such. Actually that would be a super fun companion. (you're not really a valid Magus if you refuse to cast spells)
Just in case. I was not implying that recovering from alchoholism is in any way "fun" just an interesting RP dynamic.
But recovering from "spelloholism" can be entertaining 
"Worsening" of Magic Addiction could be explained as gaining related Flaws during Bad Twilights - like Waster of Vis (needs to use excess vis to feel 'binge'), Blatant Gift (cannot stop but has some minimal mystical effects permanently on) or even Twilight Prone ("spellcast until you drop"). Totally cool!
Definetly better to roleplay it as many suggests than to just go with the mechanical flaw rules.
I think it would be interesting to figure out what kind of "buzz" you character gets from casting. What drives him to do it? Does he get impaired by it and start botching more? Maybe it gives him more energy and he can actually get more work done in the lab... at first.
Then play the transition from enjoying it, to feeling incomplete without it, to absolutely needing it. His research starts to languish as he sits in his lab each day getting high. Maybe he loses a few seasons completely.
And in the long run you'll have to figure out how he's going to cope with it. After all, I assume you won't want to go cold turkey. It's not much fun to play a mage who never casts spells. Maybe he'll rely on enchanted devices as much as possible and only cast spells when he has too. Maybe he creates a potion that makes him feel sick if he casts too many spells. I could be very interesting to play, assuming you've never had to go through it for real before.
Step right up and see for yourselves. This fantastic brew of "Spell-tabus" will ensure that you no longer cast any unnecessary or unwanted spells. Excessive use of magic makes you sick as a dog. Yes, a dog! The perfect remedy to combating your magical addiction...
I have a young apprentice character that suffers from magic addiction. In her case this relates to her side effect - whenever she casts she is imune to fear and panic (which counteracts her restriction:can't cast while paniced). With her the magic addiction seemed natural - why would she stop casting when casting took away the fear (she is a very timid person - was picked upon a lot while young).