What sleeping pad actually survives cold ground?

I finally admitted my foam pad is not cutting it below about forty degrees. I wake up cold from underneath no matter how good the bag is. I have been reading about R-values and I think I need something around 4 or higher for three-season use where I go. The inflatable ones look great until I think about a puncture eight miles in with no repair kit that actually works. So: what are you all sleeping on, how many nights has it lasted, and has anyone had a failure in the field? I care more about reliability than about shaving off ounces.