When I broke a finger and was forced to ride on the trainer last year I used an old wired Sigma computer to get the speed off of my rear wheel and (logically) as long as I was in the same gear and the resistance on the wheel was the same, it was a pretty good proxy of power.

The "data" it provided was very useful during a single training. As you progress through identical intervals, while maintaining the same "power", heart rate would go up ever so slightly on each interval.

However, I highly doubt that the actual readings were consistent from day to day, as the "power" reading depends on a lot of variables, including but not limited to:
- tyre pressure and temperature
- actual resistance of the trainer (I used a Tacx Satori, and because the resistance setting is cable-actuated, I doubt you could actually achieve the exact same resistance every day, unless you would completely fix the cable in a single position)
- temperature of the resistance unit (influences resistance on pretty much all trainers)
- room temperature, humidity, atmospheric pressure (which also influences your own performance, so you would need more or less actual watts to obtain the same "speed" on the rear wheel in a certain resistance)

Therefore, at least in my case, it was next to impossible to really track progress based on these "watts".

So although it seems to be a good concept, how reliable are these virtual power measures over the long term?

I don't think any of the variables you listed are really a problem. Most of us use the trainer in the same place each time, so the room temperature is quite constant. Tyre pressure is easy to check before the workout, like you do before each time you ride outdoors. Good quality trainers like the Kinetic Road Machine are calibrated to power and provide the same progressive power curve each time, regardless of the temperature of the resistance unit. If you are suspicious, you can always ride an extra 10-15 min warmup before starting the actual trainerroad workout; it should warmup also your rear tyre and the resistance unit.

I've been using the trainerroad setup and love it. I had been using the sufferfest videos and was happy to find them so well integrated into the software. the combination of those videos and realtime cadence and virtual power feedback gives me a much better workout that PEL alone. The ant+ usb stick and the garmin cadence/speed sensor set me back a total of $40, so it is a great inexpensive way to get exactly what I had been wanting to get me through these long winter months.

Here's another thumbs up from satisfied user. I have no opinion on the virtual power option, but i think the plans are worth the money alone. Plus the motivation to keep the power at the proscribed level and complete all the intervals makes what can be a pretty dull experience more bearable.