But they'll warranty it for anything over .75" of sag,
After almost exactly 7 years of daily use, it looks like I have maybe .25" of sag. It's not a big depression in the middle of the mattress, but a slight low spot that runs across the width of the bed about where my hips would rest.
The reason I won't buy a foam mattress is the insulation they provide. You wind up cooking in your own reflected body heat.
The Tempurpedic has a thick,
ventilated mattress cover that, for me at least, alleviates this problem. I've never had a problem with overheating.
tanstaafl.