i've never used one, but having looked at it, and having had both hips replaced in seperate operations, i would say they look like a waste of money. i just used to clamp an ordinary pillow or large cushion between my knees when lying on my side and never had any problems. when lying on my back, i would have 5 or 6 pillows to prop me up and would throw them out or add to them to change position.
personally, i wouldn't bother with a slumber support.