I have... a few weeks ago I was pedalling along and my foot came flying off the pedal -- turns out the strap had pulled out from the bolt thing that sort of pinches it in place. When I tried to put it back, the bolt seized up and then snapped off.

I've been running regular toe clips since then... still debating whether to give the Power Grips another try. I didn't crash but it was a bit scary when it failed.