Although I live in a climate where I can ride year round, I also work so I can't get out during the week to ride. So Spin class fills the void. I've been using spin for about 15 years to supplement my cycling. I just have a couple of problems with spin - 1) it's not always good for outdoor cycling - it's sometimes goes too anaerobic or the up, down, back, forth, quick spin stuff is really bad for knees and not something I would ever do on a real bike so... I generally stay with the class but opt out of some of the fast up-down stuff. I just sit and spin at a higher cadence (say 100 rpms).

Also 45 - 60 minutes is not enough time for me so I get to class 30 minutes early to warm up and then do another 30 minutes after. Thank goodness for iPods!!! I always have mine with me and hate to say it, sometimes I hate the instructor's music so in go the iPod earphones and I listen to my own.

Bottom line any aerobic sports is good for you and helps your cycling. I've done century rides after spinning for months and no actual road riding (had a fx'd collarbone).