I've ridden powergrip straps, here's what I think:

Pro's:
*affordable
*you can make them really tight
*easy to take foot out of pedal in a slow fall situation
*no clip means you can position your foot in any way you like and adjust the strap that way
*excess strap hanging off the bottom of the pedal can act as an audible pedal strike warning. if you hear it, you're getting close

Con's
*if they're really tight, your foot will fall asleep or get sore
*to adjust the strap, you have to unscrew an allen bolt. It's trial and error whether both will be even
*different shoes have different sole thicknesses. again, straps are a pain to adjust
*people have stripped the threads for the strap adjustment bolts and bent the straps hardware
*they look fugly