I have one.
The good thing about them is that they fold up and are therefore easier to get in your pack (personally, I don't like things strapped to the outside of my pack).
In truth, I think they are too thin to sleep on. But that doesn't bother me, because I use an inflatable hiker's mattress, and the German kip mats are perfect for putting underneath that to stop it getting punctured.
But I wouldn't sleep on just the mat itself - much too thin. But they're ideal as insulation against odd sticks and stones puncturing your mattress.