I've been doing spin classes for about five years now. And the number one thing you need to find is a good instructor. If the person teaching the class is a biker (or gears the class for bikers) you're much better off.
The gym I'm at now has four different instructors, and each one has a different way of doing things. Types of music, speed of the class and just over all way of doing things. Bottom line is find someone you like and it's a hell of a workout. But it's also what you make of it too.