I have back problems and usually sleep fitfully while on the trail.  Last summer I forgot to pack one for my backpacking trip and hated every minute of that decision, I barely slept and was a bear. This summer I did my homework and got the Thermarest Prolite-4.  My back has been grateful ever since.  It was over $95 at local sporting goods stores, so Ebay was my number one choice. I got one for my father and I for a 35 mile trip through the Cascades, it was the best Father's Day present he got.  He told me on several occasions that  the pad made the trip enjoyable.  I slept like a rock on it.  The difference was great. The pad absolutely blows away closed cell pads. What's even better, I adjusted the firmness until it was positively dreamy. The only thing is that I'd buy a stuff sack from another company, it'll be 2/3 the price.  I can recommend the 4 liter Sea to Summit small bag. Its super tough (I drug mine strapped to my pack down a 50 ft cliff, brushed the dust off and it was good as new) and light.