For commuting I think there is slight improvement in efficiency. I do think they help you go a little faster especially sprinting to pass someone. Overall I didn't find it to be a huge difference. I'm surprised at the percentages quoted. What I used to see floating around in response to this question was a 15% increase in efficiency. I don't know any actual data either way.

They do keep your feet from slipping from the pedals which is the main reason I use them.