I played your game, and I thought it was really good :3
I didn't beat all the hardcore levels, but I came close.
The graphics are good, even if there's nothing really special about them. The sounds are fine, as is the music. I especially like the audio at the rewind screen :D
A few things of mention:
1. I didn't really like the jumping mechanic that much. In my opinion, it forced the direction too much. What I mean is, when you press a directional arrow key and up at the same time and then let go, you'll still move much to the side. This is difficult when it comes to precision, which is probably the main reason I didn't feel like beating all the hardcore scenes XD
2. When you die, it would be nice if you could just double tap 'enter', instead of reaching for the 'R' :p
3. I feel like five seconds is a pretty long time to wait after each death. I'd say that 3 is enough for everyone to get ready :)
Otherwise, this was an awesome game. I can see how SpeedRunners has inspired this.
This isn't the first game to use a similar mechanic (rewinding), but it was a well made one!
The simple elements of the enemies and hazards worked well. Some times, you had to be pretty precise with your movements, but I managed.
Well done, and good luck! :'D