I think it was actually pretty good movie. My only real problem with it is that it didn't seem "Sonic-y" enough, if that makes sense.
I'll explain it this way: I read a lot of the Sonic the Hedgehog comics when I was younger (and really, the only reason I haven't read any in a while is that I haven't had access to them). This series - especially the later ones - had some strong characters, decent plotlines, and a fascinating world. I would have loved a movie that brought us to the world of Mobius, gave us the Freedom Fighters, the Chaotix, Dr. Robotnik/Eggman, Shadow, Monkey Khan, etc.
Instead, we had a movie set on Earth, that's basically about humans. The only actual Sonic characters are Sonic and Robotnik.
I liked the movie, and the (spoiled above) post-credits scene makes me excited for a sequel, but really, I think I'd rather they'd just made the sequel.