I believe memory foam saved parts of my life. I have severe arthritis and could never kneel on any surface in my life...apparently I did not even crawl but was a hands and feet type four wheeler *laugh*.
I am on my 3rd memory foam mattress in 12 years.
First, expensive designer name...loved it, it lasted well...eventually cut down to fit a van which compromised its integrity and it deteriorated quickly after that. It was a 10", 4" memory, 6" high density...all are combos.
Second, a Walmart brand, cut it down very soon after buying it...another quick demise. 8", 3" memory, 5" hd.
Currently sleeping on a decent brand name that has been cut down for my van...it too is compromised as the factory edges are probably heat cut and the tearing action of (I use an electric knife to cut foam...like a hot knife through butter) cutting them down exposed more foam cells. I still love this mattress, will keep it until I get another. This one is 8", 3 1/2" memory, 4 1/2" hd.
I find them very hot all year round and use a feather bed or down duvet between me and the mattress and a cotton mattress protector on the mattress.
I would recommend trying before you buy but then perhaps see if you can grab the same brand online for a deal.
If memory foam is new to you one of the keys of its comfort is moving slow, as in rolling over if its a fast jerk the impression of your body takes a few seconds to refill. If you roll over slowly it almost helps you by filling in the negative space allowing you to roll smoothly.
My experiences from my body! Good luck and a great night sleep!!