I recently have been introduced to spinning at my local gym. What I thought was a bit of a gay 'cycle to music with an instructor' has in fact turned out to be a complete lick out - not helped by a lack of air-con in the place.
I was pleasantly shocked at how much I thought that it had worked me out. Like most things, it all depends how much resistance you use on the machines and some people quite blatantly coast along.
I could be quite take by this training and do a little more.