I want one

Power readings without fitting hassles? Without high cost as well? Hmmmm. Check out iBike Pro. I'm thinking about it. Looks really easy to install. Just screw it on like a bike computer, with a wheel sensor as well. Then weigh yourself and your bike. Add that data plus altitude if known, and do some flat road tests to calibrate the thing and... bingo!

From most expensive to cheapest, your power measurement options (as I understand them) are now:

1. Power measured off the cranks (most expensive, and you are stuck with it on that bike unless you want to do some major component-swapping - but surely the most accurate way to do it)
2. Or from the rear wheel with a hub-embedded accelerometer (slightly easier to swap from bike to bike)
3. Or by measuring chain tension (using optical sensors), a bit fiddly but cheaper
4. Or by calculating power from base data (speed, resistance, altitude, temperature and angle of climb)
   

Take your pick! The calculating (vs measuring) options are certainly cheaper but rely upon the accuracy of the inputs - and some of that relies upon you. The ibike Pro works like that, as does the HAC4, although the iBike seems to do it continuously whereas I understand the HAC4 only calculates power on climbs. Feel free to comment!