I know people don't like the idea of powercranks here at letsrun and either did I.
However I was injured earlier this year and thought what the heck and started rehabbing by using them. They always felt gimmicky but when I got back to running I felt I had better form then ever before. I kept using them as part of my training.
For the three previous years I was between a 34 and 35 minute 10k runner. This year after I started using the cranks I was able to crack 32!!! That was the only thing I changed.
I put this video together with some clips of me running prior and after the cranks (you can see my body type is the same but my stride and speed has improved). It also shows some of the workouts I do.
http://www.youtube.com/watch?v=oHg5SJYRHA0
Anyway I get nothing from the powercrank people just thought I would share my story of one.