I use strava to measure my progress, but only in comparison to itself on the same route and segments.

It's a nice tool because it tracks "segments" and times which is great because if you ride the same route over and over and over you can go back and look at your just YOUR times for that specific segment or route. For example I have a hill I ride often for training purposes close to home. If I click on the segment and choose "my results" I can see back on Sept 29th my time was 16:46 at 8mph at 181 watts of power. I've done that segment several times over the months and again Saturday and my best time was 14:39 9.1 mph at 218 watts of power. So for that hill alone I can see I've gotten a full mph faster and dropped more than 2 minutes from my initial time for the 2.2 mile segment.

Although it may not be a completely real world accurate power rating when compared to itself I can see exactly how I've improved over the months. The only thing I can see a problem is if I'm heading a serious head wind on one ride and no wind on the other the numbers are going to be pretty off since I'm obviously putting down more power to get through the wind. However if there is no wind on both rides the power results shouldn't be too far off.