Just plain fun.
The graphics here are essentially the traditional 2D look with some tweaks. First of all the characters themselves are 3D models, while the background, along with the game-play itself, is still largely 2D. This provides a unique look to the game, that I personally like. It's sort of like bringing the old Sonic games into the new age of gaming. The backgrounds are all well done and very colorful, in the usual Sonic fashion. Everything is pretty traditional, with some 3D effects added.
The soundtrack is pretty typical for a Sonic game. Not bad, but not amazing either. Fast paced rock with bad lyrics and some techno. I just turned off the music and listened to my ipod while playing.
The game-play is classic Sonic game-play for the most part. Very fast with solid level design and power ups. Although sometimes I just can't shake the feelings that I'm just pressing forward and occasionally jumping so I don't fall into a pit. This is especially the case when you pick up enough speed to just run straight through your enemies. They did add in a few novelties here and there like swinging on a chain to reach an otherwise unattainable place. However, this has all been done before, and you really don't come across things like that very often in the game. You can also play as Blaze for some more game-play value. The boss fights are in 3D to mix up the game-play a bit, and it actually works quite well. Why they can't accomplish this in the console attempts of a 3D Sonic is beyond me.
If you're a Sonic fan you should check this game out. If you're just into platformers, you'll probably enjoy this as well. This is essentially what the console Sonic games should be.