For me the problem is that it's, in general, a PITA to record the data. I'd like to keep a record of my exercise mainly because it's to your body as a diary is for your mind and feelings. However, beyond a little black book and stubby pencil all these electronic gadgets seem to be both troublesome and unreliable to use as well as forever becoming more difficult to exchange data among. It should be that you strap the damn thing on perhaps with an HRM round your chest and GPS/recording device/display on your handlebars or wrist and that's it (something similar to the Nokia Sports Tracker)- it gets uploaded to something I use everyday like Google calendar. Instead, there are a bunch of dumbass cables that need to be hooked up to your computer and uploaded to some weird windows only device driver and then to some website that's liable to go out of business or be dropped soon. Then, there are two or three different devices which provide incompatible data not only among the various devices but even across different generations of the same device. I'm not talking tough comparisons like whatever but simple stuff, like how many miles have I ridden this month.
I tried all this stuff and I'm back to the stubby pencil and google calendar. Sad.