This is pretty much a GameCube port of Sonic Team's DC masterpiece. There are some significant differences, though. The graphics are a bit clearer and more vibrant, a new rendered intro has been added, the Chao minigame has been dramatically improved, and 2-player mode has been expanded. Unfortunately, the hidden Bigs have been taken out of the game for some unknown reason. The important changes are listed on this page. The rest of the gameplay info (which is the same for both versions) can be found on the Sonic Adventure 2 page.
Sonic Adventure 2