Where's a video camera when you need one?
Sure, there's a still picture of this
guy with an engine strapped to his ass, but watching this? That would be pretty sweet. Check it: it's called "the RollerCycle—a 1.5 hp engine that ups your max speed, on blades, to 25 mph." Now who WOULDN'T want one of those? Using your own legs to power your rollerblades is SO last millenium.
Then again, I also think
pogo boots would be hella fun.