I own a Tempur-Pedic Rhapsody. Great bed, great sleep, and my lower back problems have disappeared. I don't see what all this fuss is over heat-- I don't notice any difference vs a standard mattress.
For those that may have tried a memory foam bed and may have not liked it, I found out one extremely important factor shortly after buying it. I had bought it along with a platform bed w/ slats which typically do not require any type of box spring. The first few nights I slept like hell and my back killed me each morning. I called the mattress store to return it and they asked my if I had purchased a foundation with it (which I had not because of the slats). Seems like memory foam absolutely needs a solid even base to lie on, not even a box spring is sufficient. I had one of these delivered the next day to lay over the slats, and holy shit it made a night and day difference.
That being said, a lot of the cheaper knock offs are almost as good. Prior to my tempur-pedic I had the cheaper Sealy copy which was almost as good.