When I first played through this game I really did enjoy some of it, but for the most part was unimpressed. The typical DIMPS level design flaws were back in full force, the combo mechanic was implemented poorly, and the bosses were mostly pretty bad.
But I played through it with a friend using local coop the other day and had a blast for the most part. Coop really is where the game shines. It didn't exactly change any of the flaws I named, but working together to get through the levels and beat the bosses was genuinely fun... Until the Boss (and 1st act to a lesser extent) of Sky Fortress.
Sky Chase style gameplay does not work very well when the plane doesn't automatically catch Sonic. The combo button makes him teleport back to the wings, but he's usually dead before you can reach it. The fact that the boss is terribly designed and entirely too long doesn't help. Instant kill attacks at the end of the stage? Really?
Overall though it was a genuinely fun experience. I have practically zero interest in replaying single player mode, but coop I just might.