I did a few spinning classes a couple of years ago, when I was monster fit. The final one - 90 minutes - nearly killed me. Heart rate got over 200, and the sweat was flowing in a steady stream - not drips - off my chin for the final third. It was hell. Suffice to say there are only a couple of hills I've suffered that hard on, and certainly not for that length of time. For endurance it's brilliant, but it bears no resemblance to the real thing and it's essentially a kind of cycle masturbation, and not just cos spinning classes are often full of w**kers. The main thing is that if you're doing a 30 mile circuit at 20mph, you don't usually have some lycra-clad Nazi screaming 'Good Job!', 'Go for it!' and 'Make this count!' at you to a soundtrack of dreadful hardcoreacideurotrance anthems.
Anyhoo, great for general fitness and weight loss; rubbish for bike handling, self motivation and your basic quality of life.