I started spinning this winter, but I have avoided the instructor-lead classes and do it on my own. I saw too many people in the spinning classes who didn't even work up a sweat. For me, spinning is very intense, much more than being on the road. I don't understand why some are saying it does not provide cardiovascular training though. Unless I am spinning with virtually no resistance, my heart rate routinely gets to 70-80% of my theoretical maximum and higher when I do intervals. Maybe this just means I am starting from a very low baseline compared to the other riders who have posted.
Distance measured on the spin bikes doesn't have much relation to reality - it seems to be based on the number of pedal spins, regardless of resistance. I need to output about 300 watts before I feel the effort for distance is about the same as on the road, and that is much higher than I would average on the road.
Mike