I picked up Sonic Adventure 2: Battle nearly a year ago and what can I say, this is a pretty decent Sonic game for the GameCube but let me go into full detail about what I think about this game.
I like the music in Sonic Adventure 2: Battle especially stages like Pumpkin Hill, Lost Colony, Dry Lagoon, Metal Harbour (well, basically nearly every stage music) in the game. Secondly, I liked playing on the speed and shooting levels because of the sense of action I felt when I was in those stages. I also still go back and play those stages again and again because of the fun I would get out of playing those levels. The main reason why this game is replayable is the Chao Garden. Raising your little Chao, sending them to school, playing mini-games with them is a blast and highly increases the replay-ability in the game.
The downsides of the game are:
1) the slippery controls in stages including the kart stages (which seems to be a RUNNING theme in a number of Sonic games, Get it? Running? because Sonic runs at the speed of sound???)
2) the annoying glitches especially with the Homing Attack and the Light Speed Dash
3) Those goddamn treasure hunting levels :(
Overall, I actually think that this and Sonic Colors were the only good Sonic 3D games to exist. Unlike games like Sonic Heroes, Sonic Lost World or even Sonic 06, Sonic Colors and Sonic Adventure 2: Battle did not do anything stupid like make poor level design, have unfinished and unpolished gameplay or even put you into teams of 3. I recommend this game to anyone who wants a good Sonic 3D game, especially as the Sonic 3D games are not getting any better as years go by.