I have just completed an experiment. For the last four trips I used different sleeping pads. My ranking of comfort are:

1) therma-rest or similar brand air-pad -- the size and thickness really did not matter much (probably because I only weigh 115 pounds).

2) flexible blue pads with an extra for my hips

3) Z-rest -- I did not like the feel of the little bubbles

4) WallMart blue pad -- hard as a rock! even though thicker than REI flexible blue pad

The funny thing is, that regardless of sleeping pad, two Advil taken one hour before bed mititaged a lot of aches and pains. And selection of sleeping surface (find a nice indentation for hips) helps too!

I conclude that for us old folks, there simply are aches and pains that have nothing to do with the softness of the bed. I am even achy on my bed at home! Granted, the softer sleeping pads help, but do not eliminate all aches. I also find that my night aches increase with the mileage, ups and downs (ironically, more aches with down-hill) and the speed I hike each day. I am less achy at night when I take more rest breaks during the day.

I personally have had a horrible time keeping my thermarests from leaking. I have now gone through 5 (even sent two back to REI for repairs at $15 a shot). I spend about 50+ nights a year out backpacking, so I am not sure if they simply are not intended for heavy use. I do not get big holes, just slow leaks - the kind that I can never find and that require blowing up the pad about every 3 hours. Very annoying.