The way I see it, the problem that plagues Sonic Adventure 2 is its simplicity; none of the two-button gameplay has any depth worth mentioning. Out of the three shallow modes of play the game offers, the Tails stages are the worst offenders, where you lumber around in a mech robot shooting everything in sight. Mashing the B button and shifting your aim wildly takes care of almost all the baddies, and the painfully slow platforming doesnt liven things up either. The only thing that proves challenging is navigating jumps while the terrible camera goes haywire behind you.
Same goes for the Sonic levels. As Brad pointed out, they can be fun, but theyre nothing more than half a dozen little platforming vignettes scattered between a bajillion grinding rails and swing vines. In a game where you don't rotate from mode to mode like you are forced to do in Sonic Adventure 2, you eventually get comfortable with the game and the controls But when sandwiched between the agonizingly slow Tails and Knuckles levels, the pace feels terribly wrong, and by the time you get in a groove with Sonic, it's time to switch again.
If the pace was the only thing wrong with playing as Knuckles, I wouldn't have a problem with it. Unfortunately, this mode isn't just bad, it includes some of the worst 3D gaming ever. There is zero skill or challenge involved. All you can do is aimlessly wander around the insanely huge environments, hoping to run across computers that hand out vague clues about the gems you need to collect. These embarassingly bad levels are in the end what weigh the game and keep it from achieving mere mediocrity.
Sonic Adventure 2 is a case of lazy development, where a bad game is released on the strength of its name brand. I could envision this being a wonderful title with a lot more editing and refinement, but the Dreamcasts death probably doomed the game to be rushed out the door to make room for their next-gen projects. That's the only possible explanation I have for the mind-numbing Knuckles levels, other than Sonic Team outsourcing the job to 3DO. I suppose the head honchos at Sega guessed (and correctly so) that the gaming press and gamers alike would give it a free ride....after all, its Sonic! If this game was stripped of the franchise label and given an anonymous name like Kao The Kangaroo or Super Magnetic Neo, it would have received many more of the damning reviews it so richly deserves.