I used one this year because I live about an hour from the nearest pool. I got a great deal on the erg model, but haven't really used the power function. I can tell my improvement because my stroke rate increasing on the different tension settings so I never bothered to figure out the power readings etc.
It was great for me. I used it 2-3 times a week and made it to the pool 1x a week and tried to do an open water swim 1x a week training for an IM. After about 8 weeks of using it I could tell a difference in my swimming. I wasn't able to swim in open water as much as I wanted to and the 1x a week in the pool w/ the vasa got me back to great swimming shape. the last month or so of IM training I made it to a pool 2-3 times because I wanted to feel comfortable in the water (I've had a few panic attacks during swim portion of races).
It was great for me. I used it 2-3 times a week and made it to the pool 1x a week and tried to do an open water swim 1x a week training for an IM. After about 8 weeks of using it I could tell a difference in my swimming. I wasn't able to swim in open water as much as I wanted to and the 1x a week in the pool w/ the vasa got me back to great swimming shape. the last month or so of IM training I made it to a pool 2-3 times because I wanted to feel comfortable in the water (I've had a few panic attacks during swim portion of races).