I was playing this yesterday only through the first act though, I personally like the DS version a lot better. I wouldn't call this the best 3D sonic, the game is really flashy but very linear. Having the new power-ups does add a new element to Sonic but doesn't really do anything else. I like how it switches between 3d and 2d, also I'm guessing there is a lot of replay value in the game once you get the other power-ups because there were a lot of places I couldn't access early in the game.
EDIT: I wouldn't recommend anybody to buy this game for $50 but maybe a good $25-$35 if you really must have it.