Stay away from this game. Get another sonic game.
You get boss time attacks, an extra character, and a chao garden, IF you beat the game as super sonic.
"That's easy," says the veteran sonic fan, "All I have to do is get the seven emeralds." Right you are, my friend. Only one thing though: you have to get seven 'super rings' to even unlock the bonus stage. "Simple enough," you say. Oh, by the way, they're spread throughout the levels, and you have to get them your first time through, because with most of them it's impossible to go back and get. "Sounds tricky," now you say. You spend hours upon hours looking for the rings. After a few days, you find all the super rings in stage one. Now, you must get to the end, and play the impossibly hard bonus stage. "This is hard..." Right again, because you'll most likely fail some odd hundred times. After you finally get it, you shout praises of joy. But wait- 6 more to go. For each of the four characters. 7 times 4 minus the one you just got- 27 more to go. You scream in frustration and sell it at your local used games store. "This isn't the way sonic is supposed to be..."
Lemme go by that one more time:
find all super rings in a stage (must get it right the first time)
beat the extremely hard special stage (or don't, and spend more time re-locating the super rings.)
repeat for each stage as it gets gradually harder (as if it could get any harder)
Now you've unlocked a special feature. Woo-hoo.
Repeat 4 times to get everything.
I rest my case.