You may be either the Dark or Light with each differnt levels and story line. The characters you get are Shadow, Rouge, Dr. Robotink, Sonic, Tails, and Knuckles.
You also unlock SoundTracks you can listen to in the Options menu. I never really pay much attention to this though.
My favorite thing in this game has to be the Kart Racing Feature You unlock in the Tails and Rouge chase level.
This is also for two players too.
You can also get new objectives in the level you beaten in the extras menu.
Overall this is a cool game you must buy it when you get the chance.