A cheap li-lo is by far the best for camping if you're carrying stuff in a car. Packs down pretty small, generally very tough (tougher than a Therma-rest), nice and cheap, and you can use it to play on in the water too.

Otherwise just get a nice thick bit of roll-mat, preferably one with an 'egg-crate' kind of shape on the underside to get rid of condensation under the mat. Also much cheaper, nice and comfortable, and never goes down!

Grab.