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

I've not looked into P2M at all to be honest. I'll need to get new cranks as well as the unit though right.... And do any brand chainrings work with them?

The cranks depend on whether you have a pair of compatible cranks already. I did, so just ordered the P2M to fit my cranks. If you don't have compatible cranks then you can order the cranks with the P2M already fitted.

The chainrings aren't an issue, use what you have or get new ones.

Powertap calibration values: I understand 512 +/- 12 is the acceptable range. Mine is starting to creep below 500 (496 today). Do I need servicing?

172.5 on everything. DF3 has a high enough BB to get away with it... except maybe at Calshot.

Some chainrings have slight recesses machined around the bolt holes where the spider arms go e.g. FSA gossamer. These don't work with P2M. Most normal chainrings will be fine but if you're intending to use your existing rings then make sure they're flat around the bolt holes.

Why would the recess stop them working?

172.5 on everything. DF3 has a high enough BB to get away with it... except maybe at Calshot.

I've ridden Calshot on 175s on the Koga, but that doesn't make it right. Better to go down to 165s on the road bike than up to 172.5 on the track bike.

Why would the recess stop them working?

It buggers up the chain line becasue the P2M spider sits on a bit of chainring which is supposed to clear the spider on the OE cranks.

I used Powermeter24.com last August with no issues, but have read on the TT forum that quite a few people have had problems since.

I used to coach a track sprinter who had a set of Vectors and needed to ditch them as they were terrible. Max cadence limit was too low, and hard/gated efforts would throw off the calibration. For other applications they're probably fine though.

I find 165's uncomfortable. They just don't work for me (spare me the lecture on aerodynamics - I know they are 'in' right now)

Actually it's worse than that. The rings don't sit flush against the p2m, because they're machined for a spider with arms rather than a full circle like the p2m. When you tighten them down it warps the rings. Hard to describe so here's a picture that might help.

Yep, you end up with a wavey chainring. One of the riders on our team decided he'd fit his own cranks and power meter to his team bike this year but would use the rings from the supplied crank. Then he couldn't work out why he kept throwing his chain and jamming it on the chain stay. But still it was all ok, because it wasn't his bike he was f**king every time he threw his chain.

I AM NOT BITTER.

lolz. I understand the problem now. I was getting confused with the recess for the chainring bolts. Makes sense.
Recessed chainrings >>>>>

If you are buying p2m and don't get the praxis rings with it you are mad - awesome chainrings with a hefty discount when bought together with the unit

Do you know what kind of issues they were having? Was it regarding returns etc? I really want to use them but am a bit nervous

Delays in getting orders out.

Can someone inform me why vectors are shite?
Don't want to go wheel based and crank based options seem like a pricy commitment to one crank fo' lyfe.

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.

Oh, and although I rarely look at l/r balance now, looking at my data over the first few months made me glad I didn't get stages... My power balance seems to vary in different scenarios from 50/50 in TTs to almost 40/60 if I'm struggling on a steep climb.