I bought the 4" memory foam one from Costco. Both me and my boyfriend love it. We have been using it for two years. It is the most comfortable mattress ever. It really helps.
I talked to someone else that owned the 4" foam mattress and they had similar results. They also said they bought the full foam one and did not like that one and went back to the 4" one. I think the cheaper version in this case is the better. The 4" inch one is just soft enough and though enough with a regular mattress.
The only bad think is that the mattress tends to slide to one side for some reason. I thought this was because my bf was heavier than me, but when we got a new bed frame and moved everything, the mattress was now moving to my side. The other person I know that has this mattress said they have the same problem. No big deal, you just slide it over every couple of days.
Everyone that I know, that owns this mattress, likes it. I do not however like their pillows, too high. There is one memory pillow this is feather on one side that is not as high and works well for me, but that one that is designed for necks, don't buy that one. Maybe it is because I am small, but that one kills my neck.