Just finished this save for the final boss(no rings and no detectable weakness >.>). I know it's not updated for the latest version but anybody looking to play, it works fine minus a time trial mode, and multiplayer doesn't work properly. May as well share my personal opinions and thoughts.
Level design felt solid and challenging from zone 2 onward, seems to have gotten progressively more difficult and the difficulty curve felt pretty natural. Each area was impressive and very cool visually as well, I felt, when I first saw some of the zones I couldn't help but tell myself 'how cool is that'. The soundtrack was fantastic, though the music 2 zip archive is corrupted, I don't know what I'm missing though as all the levels had music.
Levels sometimes felt overly long, AEZ 1 took me 16 minutes, AEZ 2 took me 11. Of course that's the first attempt on the level but a lot of them were like this, with times of 9 or 10 minutes. I have a 'no slow down' sort of playstyle as well (resulting in many deaths in this pack). Not saying that you should make the levels shorter, though. They were frustrating at times with issues that have been mentioned in other posts, in particular the ice mountain stage sent me to the game over screen dozens of times. I mostly blame this on going too fast on top of a tall mountain covered in ice though >.>
All in all I loved it, I'd love to see this updated to work for time trials and multiplayer racing. (multiplayer hosting works, anybody trying to join crashes and the host freezes until they are disconnected again, may be a version bug or something else)