In Reply To:
All power meters calculate power; the difference is in the method.
The more popular powermeters calculate power from estimates of torque and rotation rate (cadence or wheel speed). Rotation rate is pretty easy to measure. While a bit more tricky, torque can be reliably measured through the use of strain gauges.
So there are only two items to measure.
The iBike, by contrast, measures 1) windspeed, 2) slope, and 3) acceleration. These three items are all more difficult to measure than torque and rotation rate.
The iBike also uses estimated coefficients for aerodynamic drag and rolling resistance, and assumes that these are constant throughout your ride. Where I live that is not a good assumption.
The net result, as I know from personal experience over three years, is that the iBike does not measure power reliably.
For example, if I ride my P3 and calibrate the iBike on smooth pavement and in the aero position, the power numbers will be off by 30W or more (as compared to my Powertap) if I get out of the saddle on chipseal.
In addition the iBike is tedious to set up before each and every ride, and eats batteries like I would like to eat potato chips. I greatly prefer my Powertap - I get on the bike and go.
Don't get me wrong - I like my iBike. It is a cool gadget and I still use it occasionally (mostly to get course profiles). But it is not a powermeter and I would not use it for training.