I am fascinated by Tuvok and jennyjoan's responses - for decades I have had to endure ridicule from people over the beds in our house. Our beds are a form of how they describe theirs.
It all started more than 40 years ago when my wife and I bought our present home. We stretched to the limit to make an offer and got it, there was basically no money left. I built our 2mx2m bed from wooden boards and sheets of plywood, with two drawers underneath. The size was utterly unheard of, but that was what we wanted but no mattresses would fit so I bought to measure the firmest foam (two pieces) I could get.
Over time the foam became softer and softer, foam does that, so we were now "bottoming" every time we moved. After once replacing the foam, on the next due replacement I found much firmer foam - some refer to it as Frostex. It is in fact a composite of what for all intents and purposes are all sorts and colours of scraps/leftovers in a "broth" of new foam that has set together. When I got it delivered I got cold feet, surely much too firm I thought so I laid down on it on the floor to test how it felt. I woke up 15 minutes later, had instantly fallen asleep - knew straight away I had hit upon something special.
Our sons were brought up on the same and we still have all the originals in use decades later. One lot of visitors actually asked me why it is that they sleep so well on our bed - they (he in particular) suffer from back pain and over the years they have spent a lot of money on beds/mattresses. I explained that the reason is that our bed is flat and firm. The only time I have had hints of discomfort in my back on waking up (or any other time, actually) has been when I had spent more than one night on a sprung bed/mattress. I would not consider using a sprung bed unless it was exceptionally firm and then only for a few nights (springs soften with time) - not much choice if staying at a hotel.
I find particularly absurd the tendency of manufacturers and retailers to suggest that the thicker/higher the structure, the better the sleep. The top of our original bed/mattress, still in use, is about 40cm off the floor - this was just a bit too low when my wife was recovering from hip replacement surgery (we put something under the mattress for a few weeks), otherwise it's a delight.
Incidentally, the fellow who sold me the foam asked what I was going to use it for. When I told him he in effect told me I was mad. When our son moved out he was appalled at the bed in his rented accommodation. I bought him a sheet of thickish plywood, some foam and he stored the landlord's sprung mattress. When he subsequently moved to unfurnished accommodation he bought a pine frame and carried on with the rest on top of it. He is about to move into his own home - his bed frame, plywood and foam are going with him, the new sprung "double" (actually 150% of a "single") that came with the house he will leave for visitors. All you need is 4-6cm thickness of foam, a bed cheap as chips with magical comfort thrown in.