I like the passive regen concept in Buu's case. Buu rarely has to exert any kind of effort into regenerating on the show, and when he does it is nearly instantaneous. We can't have that, now can we? With 180 hit points he doesn't need a fast passive regen. Maybe 1 every 3 seconds or something. This way the player would be rewarded for staying alive over time as opposed to just hiding out camping. The only issue I can see with it is Buu's running away and charging up mouth blasts while they regen.
You might think 1 every 3 seconds is too slow, but if you take his HP into account you'll see where I'm coming from. 180 hp transformed, +1 every 3 seconds. If you're like me, and you aren't killed occasionally for minutes at a time, that can really help you out in little ways. If you made it faster, there wouldn't be too much use for turning people into cookies.
On a seperate note, I like the concept behind immediate regeneration, but I think you should be able to stop it at any time. Maybe it should work like the Gohan shield, where the longer you use it, the longer it takes to regain control of your character (well more like how long it takes to fire the shield after absorbing a volley of generic beams of energy). With the way things are, I'd say that Cell should have the most potent regeneration, as his hit points are not much higher than any of the saiyans, and his power level is pathetic. Then again, I'm biased.