I'm taking an 8-week class at Quad right now and I'm two weeks in and loving it. As other reviewers are saying, it's a bit cramped, but really, you're intended to be on your bike for the duration of class so once you're on, it's not a big problem. Packed classes just mean more sweating and isn't that the point?
The instructors are fantastic, funny, and motivating. Music is selected to be motivating too and cranked which just makes me want to go faster!! They've got a disco ball and coloured lights set up too to keep you pumped about being there.
We did abs after our last class and that's when the cramped space becomes more of an issue, but we made it work... and it still hurts to laugh so I know it was worth staying for.