I did a few months of TrainerRoad last winter and was impressed. Granted, I've never trained with power before, but doing the various sufferfest videos with defined goals for speed/cadence/vPower seemed to make a much bigger difference than the year before (where'd I'd do the same sufferfest videos with RPE only).

Like a few others have said, it's only $10/month - that's a helluva lot cheaper than a dedicated PM (at least for the first several years)