When we say “the whole thing,” there’s only one level in it. Also, you may think this is being played incredibly badly. You are, of course, incorrect.
There’s a single level in it, as you can see below, and it appears that you can only play as 2D Sonic. The final game, which releases towards the end of the year for PS3 and 360, will allow you to play every level in 3D as well.
Take a look.