Actual power meters suffer from variance. Virtual power doubly so. The more variance in reading and lack of consitency makes it a gimmick, at best. Waste of time in my opinion.
For long efforts you'd be better off using time, HR and RPE, shorter efforts - time and RPE.
It's going to vary on a few factors; Tire tensions, air pressure and tire wear. On top of that some trainers have a drift due to heat accumulation.
Some though, after you warm them up, are very solid. It's just physics, so I'm sorta surprised by Tapeworm's reaction. Tapeworm, do you feel that the Lemond Revolution power pilot is also a gimmick? They're working on the same assumptions that we are.
On the other hand, I'm not surprised by his reaction. We haven't been doing a good job of publishing data to show how accurate VirtualPower is. We have a few parts on order so that we can hook a motor up to a bike on a Trainer and generate repeatable tests. I think when we show that people can make informed decisions on which Trainer they want to use and what they're getting out of it.
I think that if you don't have/can't afford a power meter and have a decent trainer, then VirtualPower is a superior choice when compared to HR or RPE based Training. A power meter will be more accurate, but you can use TrainerRoad for 20 years before spending the same amount of money on a power meter. So it's just a decision for people to make about how they want to spend their money.
Our existing customers think that we greatly enhance indoor trainer workouts (whether they use a power meter, VirtualPower or a HR monitor). A lot of people say they actually USE their trainer now and ENJOY it. You don't hear people saying they enjoy the trainer very often.
There are also some big names using it and paying for it, including Simon Whitfield. But, if you feel like you already have great indoor workouts, don't get bored during them and are motivated to do them then we aren't the product for you and you would be wasting your money on us.