The thing is, if you aren’t comfortable, you won’t sleep well no matter what the mattress is made of – and everyone is different. There is no one correct mattress firmness or type for optimal health and rest.
Yeah, FairyChatMom, you got it: Tempur-Pedic foam = temperature-sensitive foam = Isotonic foam = viscoelastic foam = Swedish foam = memory foam = NASA foam. Tempur-Pedic is just the highest-priced brand name (some would say highest quality, too).
As Athena noted above, the Tempur-Pedic brand mattress is a layer of their temperature sensitive stuff bonded to several inches of plain old foam. (They used to show this explicitly in all their promo materials, but they don’t on their website anymore… hmmn!) Therefore, you can approximate one for many dollars less by getting just the “mattress overlay” from TP, or even more many dollars less by getting one from elsewhere, and putting it onto whatever inexpensive base you choose.
Several companies are making viscoelastic foam now, and some of them match the TP in quality, IMHO. The key difference between all the choices is the density. TP is about 5.5 pounds per cubic foot, IIRC. That’s denser than many people find comfortable – it just feels too hard to some, though some of those find that after sleeping on it a while they like it after all. Other brands come in similarly high densities; ~4 and ~3 are common too. If you think TP is too hard you might like the lighter kinds better – the different densities feel dramatically different.
On eBay, you can get a knockoff brand, ~5 pound, 3" thick, brand-new viscoelastic mattress topper for any size bed for under $200. (You can also get less dense versions for less money. You can get 4", 2", and 1.5" thick toppers on eBay too. I’ve seen 1" thick pads in catalogues.)
We got a topper through eBay about 18 months ago, put it on a bottom-of-the-line $80 cotton futon, and now sleep much better than we did before. We toss and turn significantly less. In particular, I never wake up with my arm or foot “asleep” anymore. I also use a matching foam pillow, which has solved the aches I used to get from squashing my ears when sleeping on my side (my ears stick out farther than most people’s do).
Unexpected bonus: when your partner moves around in the middle of the night, you won’t get bounced. It’s even better in than those brand-name individually-wrapped-coils mattresses that they used to show dropping a bowling ball on, 'cause those mattresses actually DO bounce if you leave the fabric cover on them and add sheets!
Visco-elastic foam reacts to your body heat less if the bedroom is very warm already (above 85 F). It also “breathes” a bit less than a regular mattress. Here in an Austin summer, we find we need to lower the room temp to at most 85 before going to bed in order to be comfortable now – we used to get away with up to 90. YMMV.
Changing positions works fine for me – there’s only a few seconds of lag time for the mattress to reshape itself. I really like the feeling of a me-shaped nest. I find it very cuddly.
I recommend Googling around to ask for free foam samples from as many companies as you can, especially from those who specify the density. Also go lie on a TP one in a showroom if you can (and spend enough time lying there that you see what it’s like when it forms to you).