Pretty exciting stuff. I've written a lot here about the then-Metrigear Vector. This certainly looks a lot like the Metrigear version did. Of course the devil's in the details, and I know a lot of development has occurred since Garmin bought Metrigear.
It will be very interesting to see how this plays out. I know that when I'm riding sometimes I feel asymmetric, that my left leg maybe isn't working as hard as my right. But that's based on perceived exertion... now you'll be able to see real numbers.
And for tandems. I don't think there's another option (other than the Look/Polar) for independently measuring stoker and captain power. How many times have I seen a guy pulling his 8-year-old kid on a Trail-A-Bike and the kid (or kids in the case of the linked version) are slacking off? Slap some Vectors on that puppy and keep an eye on those little parasites.
For portability it's a bit of a wash. Swapping wheels (PowerTap), cranks (SRM and Quarq), or pedals (Polar/Look or Garmin) can all be done within a few minutes. But for travel, I think it's fair to say the pedal system is the most portable. You probably travel with your pedals anyway when you are going to rent a bike.
On Polar/Look versus Garmin: I think there's not much contest here in terms of ANT+ Sport or Polar's proprietary communications protocol. Big advantage Garmin. It will be interesting to see when each of the units get exposed to third-party scrutiny how they compare.
I am disappointed the pedals are Exustar Look-compatibles, as I'm a Speedplay fan, or particularly a fan of low-stack light double-sided pedals. Tyler Farrar was testing these pedals last December, so no surprise here.
Anyway, I'm sure there will a lot more to say on this...