I've yet to actually sleep in my AMK Thermolite II, but I've used it a few times to knock the chill off or as a windbreak and it's worked very well every time.

I don't deceive myself into thinking that it'll perform as well as a $500 bag, but it does work a lot better than having nothing. Plus, when packed it's the size of a Coke can and weighs next to nothing. And it's a lot tougher than a simple mylar blanket.