I've been using power measuring equipment for just over a year now and I've collected all my data indoors with a CompuTrainer and outdoors with a PowerTap, I am happy that within reasonable limits the data I have is accurate. I reckon that for the kind of things I do my 20 min figures are a reasonable guide to progress or otherwise.
By way of trying to get an overview of where I am 12 months on I took all my data and looked at my 20 minute MMP information using a setting of 7 days per point. I then took this data and popped it into Excel and looked at this incorporating 2 trendlines, a linear trendline and a 4 week moving average trendline.
Both graphs are attached, I have made some progress, that much is clear. Have I made as much progress as I might have hoped, who knows, I do feel better on the bike and I guess that's the acid test!