The virtue Transformed (Being) (ROP:M pg 47) gives some pretty nifty benefits to your average mundane; Immune to aging, no need to breath, eat or sleep, and even an essential trait.
While it has been canonically used as a mystery virtue (see Rival Magic, pg 31), I cannot see any reason why a magus would not jump at the chance of becoming immune to aging, and not needing to eat, sleep or breathe on top of that. As icing on the cake, you can pick up the gifted major quality, keep your spells and hermetic virtues, and still gain XP as normal (since being a 0 might creature gives no learning penalties). With the parma giving magic resist you have no need for an increased might either.
Am I correct in my assumptions, or am I missing some hidden flaw? Is this virtue too strong for my PCs?
With the prevalence of low-level Might-stripping spells (which I think would kill such a being, or at least make him mundane), Aegis of the Hearth (can't get in unless given a token), and the general notion that such Transformed beings are not human, i.e. no longer Magi under the protection of the Oath, it looks very risky.
But yes, goodies for Transformed 0-Might Magi abound: the No Fatigue Major Quality (2 more seasons per year!), being able to learn qualities (Improved Abilities, make two Transformed magi, or a Magi and a superb teacher Magical animal friend, you can easily get 100 and very soon 150 xp/season). I'd imagine there are plenty of Ex-Miscellanea concepts with Transformed as their Major Virtue gathering dust somewhere in the Realm of the Troupe Veto. I have seen more than one Saga basic rule prohibit Gifted Magical Friends and such, and I am a newbie.
To not break your saga with RoP:M, best have RoP:M p.31 box Character Guides defined for it. And have all RoP:M player character generation moderated by troupe and SG, keeping these Character Guides in mind.
RM p.31 The Living Goddess has been used by Viea at the end of her existence as a sorceress (RM p.44), and maybe by nobody else ever after.
Anyway it is for Amazonian sorceresses, what TMRE p.82ff Ascendency to the Hall of Heroes is for Hermetic theurgists: something to conclude an utterly exceptional, heroic and successful life with. But certainly not an argument to start a magus as a RoP:M p.47 Transformed (Human) at the beginning of a saga.
Whether a Virtue is appropriate or not for your saga is something that the whole troupe should decide together. There's nothing wrong per se in playing a character whose Death Prophecy is something that will happen 1000 years in the future. Ars Magica is a game that requires some troupe consensus about its "power levels".
I see Transformed (Being), however, in a different light than many people do.
First of all, when you say "why a magus would not jump at the chance", do you mean it from the magus' perspective or from the player's perspective? In character, it robs a magus of his quintessential humanity; among other things, it forever precludes a Divine or Faerie afterlife and "similar stuff" (like the Repose of the Criamon). This may not be a big deal for the player, but it certainly would be for many characters.
From a player's perspective: No need to eat or breathe is really no big deal for magi, worth less than a decent Minor Virtue. Immunity to aging is a slightly better deal, not per se, but because most other means to it require a magus to acquire a warping point per year (on top of spending a little time and possibly a little vis - but this is very, very little if measured over a lifetime). So basically the main advantage of being immune to aging for a magus is that he need not gain the corresponding warping, and this is subsumed in being outright immune to warping, which is probably the major advantage of being a Transformed magus. No need to sleep is not automatic. It requires a Major Quality (no fatigue), which also gives you the substantial penalty of not being able to exert yourself save by spending Confidence. This can seriously hamper your magic: you need to power every fatiguing Spontaneous spell with a precious Confidence point instead of two minutes of rest. So yes, it's very nice to have those two extra seasons/year, but if I really really wanted them I'd rather convince my troupe to have our Covenant located, or partially located, in a time-stretched regio. The Gifted Quality is, again, not something that you gain for free, but something that you must pay for, just so you can have Hermetic powers. Immunity to Warping is, on the other hand, a pretty big advantage. It makes you immune to Twilight, and allows you to pile as much magic on yourself as you want, without any long-term repercussion. But the key is long-term. With just a little care, for most magi warping is no more than a minor inconvenience for many decades. You can always become Transformed or choose other paths to immortality after that.
There are several other advantages to being a Transformed being. You can (with effort) raise your characteristics beyond the human maximum; you can more easily learn Virtues/Flaws/Supernatural Abilities and raise your Confidence score; you can more easily understand how to enter the Magic Realm. These are all neat.
But there are a number of mechanical disadvantages as well, many of them mentioned in previous posts/threads. An Aegis that penetrates your Parma keeps you out, unless you are given a token; this is a problem for young magi who visit powerful covenants, for example. The weakest Might stripper that bypasses your Parma destroys you utterly. More in general, you are subject to a number of magics that normal humans are immune to, not all of them Hermetic (Sahirs might take a fancy to Summoning you, for example, see tCatC for an explicit reference to this happening). You suffer Acclimation; while there are many ways to bypass this problem, but each requires a modest effort and/or exposes you to some risk. You cannot have any Virtues associated to Realms other than Magic. Many of your sodales no longer consider you a human being, and thus no longer protected by the Code, or by basic human decency. Finally, by becoming Transformed you are making some "Essential" portions of yourself utterly unchangeable - while this can provide some benefits, it can also be a serious hindrance: someone who is essentially a "Creature of the Sea" might be immune to many shape-changing magics (for good or ill) but can also be outright forbidden by the troupe from ever gaining xps in abilities utterly unrelated to the sea, like many Area Lores.
All told? Transformed (Being) gives some pretty neat advantages, and some pretty serious disadvantages. Crucially, it gives your SG/troupe the ability to emphasize those disadvantages just enough to make the relative advantage of Transformed (Being) comparable to that of any other Major Virtue. I've never seen it prohibited in any games I played in, and I only saw it taken once (for the sake of trying it out, rather than out of "munchkinism"). The major issue with a Transformed magus then is not whether such a magus is too powerful, but whether it's "compatible" with the stories the troupe wants to tell; in this regard, I consider Transformed magi a bit like characters with True Faith, or Demon Children.