Had a Tempurpedia memory foam mattress...was an excellent bed for about 3 years, and then I started to notice a sagging spot near my shoulders (something they guaranteed us it would not do). Returned to Mattress Firm (where we bought it) and they honored the warranty we purchased with it. We decided to go with a sleep number bed for a replacement (as I simply didn't trust memory foam of that nature anymore), and it has been an absolute dream to own. It's pricy, but we've had it now for about 2 years and I have not had sleep like this ever on any kind of bed. I know that most people can't just go out and drop the coinage like that, but I think if I had a POS mattress and I could save up my cash for the sleep number setup, I would do it in a heart beat before investing in another regular memory foam mattress.
If you do go with the tempurpedic memory foam though, make sure you get one that has a solid bottom layer of foam (that was apparently what caused our sagging issue.