The great thing about a spinning class is that you can make it as hard as you want it to be. It wouldn't matter if the entire rest of the class was comprised of gym bunnies spinning a 30-27. You can always drop to a harder gear and suffer as much as you want.

I've been wanting to do one of these classes with my own bike (w/ PowerTap wheel so as to be able to track progress). I'll look into it.

--Steve