I'm not sure how simple it is. My understanding is that to use it correctly, you have weigh yourself before each ride, and you then have to do a coastdown with the clothes you are going to wear for that ride.

Even then, the power will vary based upon road surface, riding position, and position in a group.

At $400, plus another $89 for a cadence sensor, you're not that far short of a Powertap Pro.

They may turn out to work quite well, but I've yet to see one in use, (compared to powertaps which are becoming somewaht common.) My bet is that they're not not going to see widespread adoption by people using to run a power based training program.