I really enjoyed sonic Adventures on the DC, but otherwise, I agree with everyone. After shadow the hedgehog, I pretty much gave up. The game was kinda fun, but it was not good by any stretch of the imagination.
Personally, I'd like to see another good 3d sonic console game. My only problem with Sonic Adventure was that it was too easy. Sonic Adventure 2 Battle was fun, but it didn't bring enough to the table for me. The sonic and shadow levels were fun, but the rouge, knuckles, eggman, and tails levels were far inferior to the Knuckles and E-102 missions in Sonic adventure. Also, there were too many of them.
Sonic Heroes and Shadow the Hedgehog both have one fatal flaw in their departure from previous sonic games: enemy health.
In previous sonic games, the enemies never took more than 2 hits to kill. It'd be "jump on it and it explodes" or "jump on it, and it goes into some 'weak state' and then you jump on it again for the boom." Or spin-dash into it, or homing attack towards it, or something like that. That allowed for the blistering speed of the sonic games, no one enemy took any time to kill, so you could burn through thousands of the little buggers. Having to hit any non-boss enemy a substantial amount of times to kill in a sonic game? Unacceptable. I haven't played any games since Shadow, so I don't know if they fixed that particular problem yet. Not saying the games would be perfect otherwise, but that one thing really really pissed me off.