I just listened to Allen Lim's 3-min. advertisement for PowerTap and here are a things I don't quite agree with:

1) Hub is the most accurate place to measure power - he says that you don't have any chainring weirdness to futz around with when you're measuring at the hub, but neglects to mention that measuring downstream of the drivetrain introduces drivetrain losses, which are highly variable. If I take a PT wheel and use it on multiple bikes and get different power readings - is it because I'm actually outputting more/less power or are the drivetrain losses different on each bike, or both?

Not saying that measuring at the crank spider is foolproof or better, just saying that measuring at the hub also has its own issues and isn't any better or worse.

2) Auto-zero when coasting - he's wrong when he says that PT is the only power meter that auto zero's when you coast. SRM and Power2Max allow the same (with SRM you can turn off auto zero on/off). Quarq allows you to auto zero by backpedaling a number of times.