I've had memory foam for a year or so and love it. It is definitely warmer than a sprung mattress - I never have to use a hot water bottle any more, even on really cold nights, so if you are one of those that get really hot in bed you might not like it.
It is very warm, we got a single memory foam mattress for our dd's bed but she gets far too hot on it, I slept in her bed the other night(she has dragged out the co t bed mattress and sleeps on the floor she hates it so much) as dd was ill and I roasted. Usually I am quite a cold person
Sorry, a bit rambling, very comfy but very very warm!
I had back problems which are very much improved since getting a memory foam mattress, but I'm one of those that gets very hot in the night. I havent had a full nights sleep for ages because of this, except when Ive given up and gone to sleep in the spare room on an ordinary mattress. So much to OH's disgust one of our first big purchases this year will be a new mattress! Shame because it is really comfortable.