I've had my memory foam mattress for over a year. It's 10" thick of which 3" is memory foam. It's a double Highland 10" 65lb m2 (they are now better quality at 75lb m3) and I paid �82 on ebay. All the technical info is important.
Before I got this I use to sleep with 2 x 15 tog duvets on. It's true they memory foam retains warmth but has never been a problem for me (on my own) and now I only use 1 duvet.
I LOVE IT.. the cells do break down over time but that cant be avoided and happens to ALL memory foam. My mattress is still firm but rather than suck me in and keep me their, it's more flexible and sponglike. It certainly hasn't gone lumpy and is still firm and in shape. (But then again I've only slept in it :-( and nothing else)
I wouldn't buy anything less than 3" of memory foam and you won't regret it.