I was really into Spinning for a while. (I was a long time road cyclist when I picked up spinning.)
It's great for getting and keeping in shape, but as others have said, it's not riding. Did it for a number of years but had my fill and about a year ago I dropped my membership and went headlong into riding year round. I figured with what I saved in the gym membership that would MORE than cover buying some good cold-weather riding gear.
My fun spinning-related story occurred the Spring after my first winter of really attending Spinning religiously. From a cardio standpoint I was in pretty darn good shape. I'm outside on a ride on the first really nice day in Spring - so there were relatively lots of riders out. Pack of 3 riders came up to me at a stop light. They were uber-cool. So cool they couldn't wait for the light to change, so they ran it. I waited for the green and proceeded. I saw them up ahead, a ways.....we were just starting a 4 mile long stretch of open road. Had my heart monitor on. I knew what heart rate I could sustain. I up-shifted and started hammering. Kept an eye on the HR, and knew how high I could go and sustain it. I just kept hammering. As I approached the pack I checked 6 (clear) pulled into the lane and just BLEW by them. It felt great.
I wouldn't have been able to do that had I not been spinning. It's very intense. Can be fun if you make it so (get in the zone, find instructors with similar music tastes).
Jim