Anyone have experience with trainers with power? the kurt kinetic trainers ahve an optional piece for $80 that gives you power readings. Anyone know if it's accurate enough to be used for power training purposes?

Nashbar also has the "Watt Master" trainer that includes power readings and they also have another model on their site that supposedly provides power outputs. I'm not looking for anything perfect, but just want to do some basic tests more so than anything (power at threshold, for example)

Need to buy a triner because my gf is having her knee scoped and she'll be riding it for therapy so I figure since i want to get more serious with training during the winter may as well invest a little more in the power features if they are useful.