I don't play as buu, but after some experimenting with him, it seems to me like he's the best character in the game...
PROS:
He's fast. REALLY fast. The fastest actually. Making it easy as hell for him to confuse his opponents by zooming about at swoop speed, as well as awesome for juggling opponents after the first melee hit. He has a massive explosion type beam (mouthbeam), he has a HUGE ass amount of HP, if you knock someone down so they can't get up and candy beam them it's GAME OVER, and with the candy beam and regeneration he can gain all of his massive amount of health back with only slight difficulty. Plus he starts out with a higher PL than anyone so he wins all the early power struggles, and possibly all the later ones if he's played right...
CONS:
The transform is kind of late, but if you're good and just keep making donuts, you might even transform before the gokus and the vegetas do (not much of a con...). His transformation amplifier is 1.5.
But man, PL doesn't mean that much anyway; he can totally DESTROY at melee, that's where it counts, but at the same time he has that nuclear missile type mouthbeam attack, which can blast away up with the kamehameha's and final flashes.