I did a spin class on a cruise ship once. It was 45 minutes of a woman yelling at us to stand up and pedal like hell, sit down and recover, etc. rinse & repeat. It was a great workout, really got the heart rate up. But in the end I decided this was a good way to make me hate cycling, turn it into a real chore. I can motivate myself to work hard & know when I need to rest, but I can understand how someone else might really enjoy these classes.