I played classical guitar for many years using a footstool before I discovered knee-rests, and I've never looked back; they're so much better for your back.
This is the second Gitano I've bought, so I can leave one in my guitar case ready for gigs. I've tried a couple of other makes but this is the only one that folds up against the side of the guitar when not in use. It's also very light. It doesn't look too robust, but I've had the first one I bought for about eight years and it's still going strong - I've changed the suction pads just once in that time, and it's been used regularly!
My only reservation is that, if you tend to lean on the guitar a bit when you play, it occasionally pulls off. This only happened a few times at first, so I must have learned not to lean pretty quickly!
An essential aid to playing, especially classical guitar. Go on, treat your back!