Have never used it. It's a great concept - you use your own wheels and you're using the motor at peak efficiency all the time. It comes down to that whining noise. I have yet to hear it, but if it's too loud, I would be reluctant to get it. But that's just a personal matter.

I have heard that the freewheel crank is somewhat on the cheap side and may break if you put too much strain on it. But perhaps that was just a bad batch.