They definitely extend the comfort zone on a long ride, and for that reason they're worth their high cost..
They attach with nylon "belts" or straps, under the seat in 2 places.
While it is possible for it to slide off, if the belts are tight it likely won't happen. It's never happened to me personally, but I'm not pulling high "G's" in my cornering either!
The fabric cover has a rubberized bottom that sticks pretty well to the seat without sliding.
I'm on my second one in 7 years, and I'd give them 4 out of 5 stars only due to the possibility of a puncture, and after so many years the rubber "cells" inside will develop leaks. For what it's worth, they do send a "patch kit" with every new purchase.