Very nice...
This is the best flash video game clone I've ever played. The fluidity and the speed just about perfectly match the original Sonic. It's really freaking amazing, and you did an awesome job coding the controls, enemy AI, everything!
There are only two things I find wrong with it...
1. When you get hit, there's no period of invulnerability... so in a lot of cases, you don't recover from a hit at all, you just take two hits... one to lose your rings, and one immediately following... and die instantly.
2. There aren't any power-ups! No shield, no shoes, no 1-ups (that I saw, anyway...). This isn't really a major problem, the game is awesome enough without them, but having them in would make the game feel 100% authentic.