It was fun and plays a lot like a classic Sonic game! The challenge ramped up quite satisfactorily to keep even casual players like me 'til the end! I also really love the special stages--some of the most fun emerald stages I've ever played. The music is also pretty awesome.
My only gripe with Sonic Superstars is that it seems like a little novelty game to hold people over for a future Sonic release. I had a lot of fun playing it, but it didn't leave as strong an impression on me as previous Sonic games have. If Sonic Team could take this gameplay and design and give them an epic story like in Sonic 3 and Knuckles, you'll have a landmark Sonic game.
Worth a buy, especially if you are a Sonic fan!