I clip in and unclip all the time during a ride. Always release one foot when I'm coming to intersection, see a dog or child on bike path etc, anything I can see ahead that may cause me to have to stop quickly. I'm so used to doing it, that don't even think about it anymore.
I have never tried PowerGrips, but I'm happy with my pedals, so will probably stick with them.
Last time I rented a bike while I was away, I hated not being able to clip in. It felt like I lost that extra kick you get from pulling on the pedals.