Game-balance wise, it's iffy if done right (i.e., by a Wiz17+ or Sor18+ who can - if s/he has enough xp - cast
wish over and over):
At these levels, you can gain 3-4 feats instead of a level - and then, also advance in levels (and even in xp, if using the FRCS xp rules) much faster than the rest of your party - whereas, say, a fighter gains only one feat per two levels (as well as HD and skill points, and better BAB and saves).
Of course, after a few levels, the rest of your party can start picking up
Epic feats - while you, being a couple levels lower, can't. (Side note: And even at level 21+, even a
wish shouldn't enable you to get extra epic feats IMNSHO. OTOH, some epic spell might...

) Also, you lose out on caster level, more spells/day, saves, skill points, class abilities (e.g., from a prestige class), etc.
So all in all, it could balance out...
Another comparsion: There is a spell (
permanency) that lets you "buy" something similar for xp:
Permanent spell effects.
The cost seems to be 500 xp per spell level (although, in the PHB, you can only "buy" spells of level 3 or less for yourself, and even less for other creatures)...
Still, there's only a small list of spells that can - officially, at least, you can (and should) always allow some others as well - be made
permanent this way, plus some more in Tome & Blood.