OMG! So many memories! And it works just FINE! Excellent flash!
I've seen some bugs, like in the boss fight in Zone 1 when you are running suddenly you stop, like there is a pixel there that you stomp into, I've been hit a lot of times because of that, the same in some up curves. Also, I'd like to see the little animals jumping over when you kill a monster.
And like DoctorThunder said: "For something from 2004, this is really good. It [...] hasn't aged [...] compared to the other Sonic flash games out there."