Power Meters / Powermeters (SRM, Powertap, Quarq, Ergomo, Vector, Stages, power2max, P2M, 4iii, InPower, Cinch)

as opposed to one pedal for life!?

Pedal strike damage potential
Finicky calibration
No pedal choice (and poor pedals)
Expensive

http://www.active.com/cycling/articles/the-pros-and-cons-of-5-power-meters?page=2

They work fine and are accurate. When they're working.

They're quite fragile. They have a tendency to not work for no reason sometimes. The pedal is shit, it's exustar and crap. The grease leaks out of the spindle eaisly as the washer seal degrades super quick.

They're a good accurate, reliable powermeter in a bad product.

Get a power2man. Honestly they're a good price and a solid product.

http://www.bike-boy.com/2011/08/garmin-vector-power-meter-review.html

I can't pretend to be an expert, and I've only owned one powermeter so no means of comparison, but I like my vectors. I've had them for almost a year and haven't had any problems. Broadly speaking I'm frequently concious of their expeniveness and so do try to treat them gently but accidents do happen and I've ridden them through flood water, changed them between bikes 2x per week in the summer, and dropped the bike heavily on it's side once and not had any noticeable problems. That's not to say that others haven't had troubles but I've been happy with mine. If I was buying again I think I'd go p2m but that is based almost entirely on the recommendations in this thread, rather than any fault with the vectors.

There's very little reason not to commit to and own a 24mm PM crank. They fit pretty much any frame under the sun, even Trek's stupid BB90, and take a couple of minutes to swap between bikes.