Might just be a one off but the one and only spin class I went to was mental! Situated in box room off the main sports hall (yes where they store the gym gear) ten or so spin bikes, no windows or fans and a bunch of keen spinners without any brains between them. No checking of owt from the instructor, just the speed of his music! The off, a madness never to be repeated by those with a preference for life... short warm up then spin like xxxx, stand up and spin like xxxx, do press ups while you spin like xxxx, sit down and spin like xxxx, easy, easy then do it all again but against increased resistance, then do it again as a lower resistance but faster. The driving force was the beat of the music which the instructor adjusted with alarming frequency. In that box room the volume of sweat leaving my body was only exceeded by that volume of blood being pumped around my body. Somehow, for me, it just didn't seem to fit anywhere within a sensible and progressive training plan designed to achieve a cycling goal. But that's my one and only experience and I've no desire or inclination to repeat it. It was absolutely a crap way to spend 45 minutes on an excuse for a bike undertaking a series of pedalling actions that have little bearing on what I would be doing on a real bike.
Sorry, rant over! I'll get my coat...