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They are warmer - I am cold nature so that is not a problem. I think they can partially be defeated with a mattress pad. No problem with bedroom aerobics.
My king sized temperpedic (standard) was painful to pay for, but worth it.
You shouldn't use anything but a top sheet over the bed. Using a mattress pad to insulate you from the collected heat also prevents the memory foam from ... memorying.
Make sure to find a store that has them to try out in your area. Some of the ones that look really comfortable aren't, and vice-versa. I ended up with the Tempurpedic Celebrity.
Someone mentioned them being really heavy. This is the truth. In addition to getting someone to deliver it, make sure that the bed frame you get has enough room between the headboard and the matress to tuck the sheet into. Especially with the pillowtop on it, the sheets barely fit, and not being able to tuck them all the way down into the crack means that they come off the bed all the time. It is probably my only complaint about the mattress.