Someone had left a GCSE Physics one on a desk in a Physics lab the other day, on the back it said "CGP: We know how goats think. If goats used revision guides, they'd be CGP."
I think it depends on the subject. They were really useful for most things at GCSE, although I found the English & English Lit ones didn't really go into enough detail. I've only used them for Geography & Physics so far at AS and those ones are quite good, they cover everything we did in class quite well, although a couple of areas are lacking in detail, whilst there's loads of detail in others.