Woefully innacurate, but pretty consistent.
I've used Virtual Power on rollers for about 18 months and recently switched to using a power meter. FTP has gone from 369w to 290w. It was clear from the start that VP was obviously a huge overestimate (I'm clearly not capable of 5.3w/kg) but 52x11 @ 95rpm produced a steady result in terms of both VP and HR. I always pumped my tyres to the same pressure right before each session too to make sure that wasn't a factor.
So it's a decent training tool I reckon, though it's a lot simpler now I'm using real power.