I used them on my commuter bike for a while. I bought the straps and added them to my existing pedals, rather than buying the strap-pedal combo. FYI, there are two sizes of straps; if you have large feet you might need the extra-long straps.

Overall, I'd say that PowerGrips work very well. Which is to say: they hold your foot pretty securely. Almost as securely as clipless pedals. If I stuck my foot under the PowerGrips as far as possible, then I had to twist my heel outward in order to get out of the strap. Unlike clipless pedals, however, if I knew I might need to get my foot off the pedal quickly I just didn't insert it under the strap very far.

All in all, I'd say that PowerGrips work much better than traditional toe cages and straps.