Besides featuring a great game overall, SA2 really allows you to play the game any way you want, much like the best of the 2D Sonic games. It does this with an incredibly well-designed replay system featuring tons of variations on level selects and a wide variety of emblem games. There are 5 missions in each level, and you get a ranking for each one you complete. E rankings are easy to complete for even a beginning gamer, and A rankings are a challenge even for a master.
The basic concept is that you play 2 differet sides of the same adventure - Hero and Dark. Hero Side includes Sonic, Tails, and Knuckles, with Amy as a non-playable story character. Dark Side includes the now-playable Eggman, and 2 new characters, Shadow The Hedgehog and Rouge The Bat.
Sonic Adventure 2 has been rereleased for XBox Live Arcade and the PS Network in October 2012. In the new release, it is now HD (720p) with also the addition of leaderboards. It can be purchased for $9.99 on PSN and 800 MSP for XBox Live Arcade. For an additional $3/240MSP, you unlock the Gamecube version.
It is noteworthy that in this rerelease, there is no new way to migrate your chao in or out of the game. Those features have been entirely taken out.