I used to do spinning classes and, while I can't say that I enjoyed them, I've never worked as hard in 45 minutes on a real bike. At the end of each class I felt that I couldn't have gone on for another minute.

It does depend on the class leader, and also on the other riders, especially when you've got fit cyclists half your age alongside you using it as part of their training regime.

No substitute for real cycling, but a great workout.