I used to do spinning classes and they do work- Get the cardio vascular side working and it is surprising how the muscles improved over a 3 month period. Only draw back against them at my gym is that they are structured for the average rider, and if you are very fit, they are not hard enough for you, unless you put a lot of effort into it. I felt that if I was doing that much effort, I would rather be riding.

This year I am trying night riding on the hills in the week, and so No necessity or time to go down the gym.