The level design is fun. Adding tails is not a joke like it was on sonic 2 back in the day...
The music is not bad, of course most levels have new music, but if you listen closely you may recognize something from a previous game. :)
Multiplayer is fun if you and your partner have a solid connection. Even though you cannot communicate, every player of sonic knows what to do and how to work together. If you don't have a solid connection the game runs like "Time Break" (If you remember back from Sonic and the Secret Rings.) It will run slow and sluggish and your stuck with it unless you can figure out a way to quit.
Also, back to the level design, most of the levels are challenging but you should be able to get through them if careful on the first attempt. Some are long, so I don't expect to see many 1 minute or less speed runs.