I can vouch for this. I've recently started sleeping on the floor, and at first my back hurt some, but after a couple of days I got used to it and now it's feeling so much better. I have really poor posture and constantly slouch when I'm sitting in my chair, and so usually have back issues. Sleeping on the floor has worked wonders.

Even when I'm not sleeping on the floor, I prefer firm mattresses, as soft ones are horrible for my back.