I like Strava and use it to document the vast majority of my rides. Don't know if I'd go so far as to call it "great" but it is probably as good as anything out there. Variances are expected when power is estimated but a nearly 100% variance (58 > 109w) on the same short course under the same conditions at virtually the same speed (10.2 and 10.1 mph) isn't an estimate it's a wild arse guess. What algorithm could they be using that inputs the same bike/rider weight, the exact same elevation/location, and virtually the same speed and comes up with the power "estimate" for the lower speed being nearly double that of the other pass? I could understand it if there was some glitch where it misregistered the speed, elevation, etc. but all the other data is there and matches so you would expect the results to be at least similar.

Strava's "estimates" on another part of the same ride had me cranking out 1,227w at 19 mph on a flat, straight section of road. I know the speed at that point was incorrect, but even if I had actually been doing 19 mph at that point, there is no way the calculations should come up with over 1,200w for my weight, on that road, at that speed. I have ridden that road at that speed and estimate my power output to be more in the range of 200w and I'm willing to bet my off the cuff guess is far closer than the results I got on Strava yesterday. When power is estimated sure there is an acceptable variance, but on this occassion and quite a few others, I have noted that Strava hasn't even been close.