My Big Agnes pad started leaking this summer, I never could find the leak. I'm a Neoair/ torso length Z-rest fan myself.
I had two of the Big Aggy Aircore pads fail before I went to the Exped Downmat. They like to leak where the valve is glued into the pad. On a couple other backpack forums, this seems to be common knowledge.
From about mid September on, I use the Exped, with a space blanket under it (usually folded in half) I've been going to try the two pad method for a long time, but what I do has been working for me. I usually spend some time kicking rocks, twigs, and dirt around where I plan to sleep before I pitch a tent on it. I've had "leakers" but I've never popped an air matress in the backcountry.
