Here's a ride segment from my ride the other night wearing my PowerCal HR Monitor/Power Meter estimator. I was pretty happy with the results. I'd say the power numbers were a tad high, maybe 5% high, but that's not a problem since I can adjust the power output (across the entire ride) using Golden Cheetah software (open source). Just go into the main menu, choose Edit then choose Adjust Power Values and Enter "-5" (in my case) and it will adjust the power profile 5% lower for the entire ride.
Here's my ride profile: