I don't think it belongs in E&RE, unless Oddwarg actually comes here and posts about it himself, but feel free to move if necessary...
Well, as for the game itself... I'm impressed! I'm still wondering how a lot of the special effects it does is implemented, but then again, the latest version of Game Maker I used was 5.3A
I think it's nice having a game like this, I always wanted to make a seamless Sonic game with no results screens, fade-outs, etc. I wouldn't say the lack of platforming was a bad thing, as this game was built to show off special effects and that's really what bosses are for. In any case, I got a huge thrill from playing it, much more than any normal Sonic platforming.
My favorites are Drill, Robot, Egg and Battle. Not the most appropriate names I'd say, but that's what they're called in the level select! Drill fair enough, but Robot and Egg could be applied to pretty much any Sonic boss... and Battle could be applied to anything!
I'll also post a link to a rather long comment I posted on the creator's deviantart, where I mostly talk about physics. Quite glad he took my boss fix ideas on board, it took the annoying bits out of Drill and Egg.
A last note, I do like the music in this game, but a lot of it is midi files with the usual rubbish instruments... I don't know about you, but I would kill for some YM2612 ports!