A lot of people state that Flawless Magic is "overpowered". Some don't go so far, but consider it as the "strongest" Major Hermetic Virtue (including David Chart). In my experience it's a good Major Hermetic Virtue, but not really that exceptional. Sure, it's stronger than Secondary Insight. But it's also weaker than a Major Magical Focus, in my opinion. This thread is to help me understand what I am missing.
Flawless Magic should be relatively easy to "analyze" because it does not allow you to do stuff that normal magi can't do (spend half as much vis on rituals, for example) -- it just gives you bonus experience in a restricted field. Thus, its effectiveness can be roughly measured by a) "oomph": how much extra experience it gives you per season and b) "breadth": how wide is the area of magic it boosts. The two are interwined, in that something that gives you many xp/season in a very narrow area is limited by how much time your are going to put into that area before you hit diminishing returns.
Flawless Magic provides 5 extra xp whenever you invent a new formulaic spell, and doubles the amount of xp you put into mastering spells. This appears a lot, prima facie. But it's balanced about how much time you spend inventing and mastering spells. I think that 1 season/year spent inventing and/or mastering formulaic spells is probably more than even a specialized magus will spend. I mean, you probably don't want to spend more time on that than on studying the Arts, or on learning every other ability (from arcane to mundane ones), or on doing everything else (training apprentices, non-spell labwork etc.). So that puts the gain at about 5xp/year, possibly a little more (say, 7-8) if you can get many good books on spell mastery -- but then, how many are there, on the spells you care about? Even if we go by the 30xp/year guidelines, and assume 1/4th of that is spent on masteries, that's 7.5xp of bonus. So, we are talking about some 900 xp for a focused magus 120 years after gauntlet. (Note that if you plan to play a newly gauntleted magus in a one-shot or for a very brief saga, Flawless magic is just not worth it except for the most one-dimensional characters - pick Mastered spells and/or other xp-granting Virtues instead).
Let's look, instead, at an Art specialist. Ars Magica suggests that, 120 years after gauntlet, such a magus might have a score of 40 - i.e. 820 xp in that Art (note that this is a little less than 1/4 of the magus' time, at 30xp/year). An affinity is worth at least 1/3 of that, possibly a little more, because of rounding effects. So ... 300xp? Spend 1 season/year learning from books with Book Learner, and that's 360xp. Remember those are Minor Virtues!
Let's look at a magus with a Major Focus, again at 120 years of age. Again, this requires a little fudging, but I think it's fair to say that on the average activity within his focus he might have 30 in his strongest Art, 20 in his weaker (that's a bare 675xp, or 22.5 years at 30xp/year). The focus provides him with a +20 bonus -- the equivalent of a score of 35 in each. Sure, it's not quite the same, because you can't use as much vis, for example, or right high level books -- but then, 35+35 would be the cheapest combination. 30+20 costs 840 xp less than 35+35.
Note that, in my opinion, hundreds of xp spent in boosting your focus Te-Fo total from 50 to 70 is more broadly useful than the same xp spent on masterying your formulaic spells. This is a very subjective judgement, but I've seen far more PCs doing the former than the latter.
Summarizing, Flawless Magic seems a strong, solid Virtue for a niche magus -- one who specializes in Formulaic spells -- but really no better than similar power-but-not-breadth-boosting Virtues, some of which (e.g. Major Magical Focus, or Life-Linked Spontaneous Magic) are probably a slightly better buy, infofar one can compare apples to oranges. I would say that the crux lies not in Flawless Magic itself, but in the fact that spending a lot of xp on spell mastery is not really that useful (a few here and there can be, though).
