Hello. Long time reader, first time poster.
I have a background in exercise phys (BS), so I have been able to read and understand many of the arguments presented by both sides of this argument. What I suggest is that you thoroughly read some of the internet arguments between Frank Day and Andrew Coggan, PhD. You can find them by searching the newsgroups with Google, and even some on this board before Dr. Coggan stopped posting here. You might also get on medline and read up on some of the studies involving elite athletes and pedalling technique etc. You will find that there are lots of holes in the supposed workings of powercranks.
It is not easy reading, by any stretch, especially if you don't have a background in the stuff. This is to Day's advantage because his arguments seem well thought out and based in science, and he does not use much "scientific terminology", in his posts, if you get my meaning. Also, his arguments seem to be based on common sense, or what has become "common sense" because it is what is fed to us by people like Carmichael and Friel, who have basic misunderstandings of exercise phys. That is easy to do. They have books out and they seem to make sense. I mean, it was a long time before I actually said to myself, "Gee...I probably shoudln't believe this works just because someone says so." and started reading.
Frank Day's arguments are not based on a good foundation of exercise physiology, or knowledge of pedalling mechanics. That doesn't mean his cranks don't work, maybe they do and maybe because of some new mechanism heretofore unknown to science. But the research seems to show that pulling up on the pedals, or the ultimate round pedal stroke, or whatever, is not nessecarly correlated with success or strength or power or success as a rider.
Powercranks kick your ass because they are training you to do something that is unnatural for your body: overusing your hip flexor muscles. They were not meant or built to work in the way that powercranks supposedly teach you to use them on a bike. If are able to, read a little bit about length/tension relationships of muscles. Muscles are meant to work best at very specific lengths and in very specific ranges of motion. For example, knee extensor muscles (quads) were built to help you walk and run, and they work great for biking because you are using them in a range and length that are similar to walking and running. Your hip flexors are not in an appropriate range, and even when trained cannot hope to generate near the power of your quads.
You don't have to believe any of what I just wrote. Read up on it yourself. If you can use the internet you are smart enough to figure this stuff out on your own. Don't let testimonials or money back gaurantees sway you. That is the beauty of science...the ability of the human mind to make predictions and understand things based on pretty simple experiments.
I will leave with one more comment regarding the benefits of PC's., or any other supposed training aid. It is the concept of the placebo effect. In the 70's there was a study where weight lifters were given an injection of what seemed to be steroids, but were really just saline injections. The weight lifters were told that it was steroids. Other weight lifters were given nothing. The ones who thought they got the steroids improved their maximal lift by something like 15% in a week.
think about it.