I've been using Trainerroad for over a year now. Love it. I've become their unofficial shill, actually.

The main caveat to using it: It's indoor training, so if you hate indoor cycling, you either have to change your tune quick, or fugetabout it.

Other than that, I love it. I actually do most of my training indoors, (M-F) so it works perfectly for me. The concept is stupid simple, but they've really make a sweet interface and good tracking of your performance. When you use it with a Sufferfest video, it's insane how good (and how hard) it is.

The good thing about TR is that you actually DO the workouts. It's one thing to read about FTP, NP, TSS, but a whole different thing to actually execute and do proper power-based workouts. I'll wager that the vast majority of people who own powermeter just use it as a fancy recording device that doesn't change their workouts much. WIth Trainerroad, that will definitely not be true - you'll be doing the FTP test, and then doing workouts based on it. It's very eye opening to see how hard 70%FTP, 80%FTP, 100%FTP, and 120%FTP are and how long you can last at those power ranges. For me, it meant more suffering than what I was used to.