I have had mild back pain all my life and often felt stiff getting up in the morning. A while back I moved to a yoga mat (both to be more minimalist and because I thought it would help my back) and missed the padding for a few days, then I was fine. I then started sleeping on the floor with only a sheet under me and it now feels even better. The only two things I feel I need are a normal sized pillow and I do like to use one of those large bamboo pillows that you hold between your legs while on your side.
Right now when I sleep I feel some sort of relief as my back flattens against the hard floor.
For naps or lounging like sitting on a couch reading with a dog, I like them really soft.
What about you guys?
When I'm backpacking I use a Z-lite or a Therm-a-Rest, they do the job, but I would not want to sleep on that everyday.