A spin bike can be great for training but should only compliment your regimen and not be the only form of exercise or cycling. I had a Schwinn Spinner Pro and once you got the weight moving, it was just spinning and not much quality effort but was great for cardio. I started watching spinning video classes on youtube and following along with those and they made me work a bit more than I did alone. I have a magnetic spin bike now and it is a much harder workout as I am doing the programs and the resistance changes automatically.

I like a combination of rollers, trainer and the spin bike and mix in a few real rides in the cold as well as running and core work. You need gym workouts to build muscle and to offset the riding season muscle deficiencies that you built up.