Really cool interpretation of the song, I like it. And this Robotnik's theme rocks.
IMO, you should just change a little the rhythm guitars between 2:01 and 2:36, they sound repetitive. At this moment, the song seems to lose "power". The first part sounds very great and this part isn't as good as the beginning of the song. I'm sure you can change them to make it rock. Everything else sounds kinda good to me.
Keep at it, man, it's great. (Oh and if you need some drums, just ask me ^^)