My brother has a foam mattress in his guest room. He found it on Craigslist (it was someone's unused guest bed), used it himself for several years, then replaced it with a larger spring mattress.

I can't stand sleeping on the foam mattress for more than a night or two. When I wake up it's like I've sunk into the foam and have been immobilized for hours. People are supposed to be able to move when they sleep.

He's since gotten an air bed for the other room, and I choose that one when I go to visit.