Amazing, but you missed something...
Absolutely amazing game, but one rather annoying thing you seem to have missed. In the official games when you hit an enemy or land on spikes or whatnot, you loose your rings, but for about 3 seconds sonic/other characters flash and are invincible, so that they can grab some more rings and get out of the danger zone.
Because your game doesnt have that, on many occasions I've landed on a big set of spikes, lost all my rings, not got any on the way up/down and fell straight back down onto the spikes, killing me off. Very frustrating!
One other thing, maybe you should put some checkpoints throughout the acts?
Other than that, great game, keep it up!