Just played through the demo. Managed to get through both levels without too much trouble. Music's rather nice, reminiscent of Sonic CD. Level design, I think, is vastly improved over SCD, and much more 'professional' than most fangames. Credit to the engine developers, the game is surprisingly solid - apart from that spindash-slope error, I didn't find any other bugs.
Only real problem I had was with the first handglider - seems like you run, jump onto the spring, and then somehow should catch the handglider. Doesn't work for me - usually end up crapshooting it. Also, the opening movie is a bag of balls. Either get a new intro or leave it out, please.
Other than that, this is easily the best (and most professional) fangame I've played in years.