I have a 4-inch memory-foam topper. For the first year or so, it was incredibly comfortable. Since then, it's lost a LOT of its firmness and support, and I often find it uncomfortable. For example, if I'm lying on my back, my butt will sink all the way through the mattress topper and be resting on the mattress itself. Then, if I roll over to my stomach, my midsection will be about 4 inches lower (because it's in the butt crevice formed from sleeping on my back) than my shoulders and legs, which is quite uncomfortable. Sometimes I'll get up to use the bathroom just so the memory foam has time to spring back to life, and then it's only somewhat as firm as it is at the start.
I have no clue if my experiences would transfer to a memory-foam mattress, but there's no way in the world that I would ever buy one. The way I see it, I could try out the mattress in the store and love it, and then after a year of using it I could absolutely hate it. I have a cheap, uncomfortable mattress right now and my wife likes the memory foam, or else I'd just get rid of it.
BTW, to curb any fat jokes, I'm 6'1" and 180 lbs.
Oh, I just remembered one more thing. The firmness changes with temperature. In the winter we sleep with the temperature being 55-60, and the topper is quite firm. During the summer we sleep with it 71, and the topper is not very firm.
Here's a
link to the topper I use.