Thanks guys.
I sometimes have lower-back issues, so I'll try a few more out. Good point, Stanley, I was actually going to ask about what they're like when you're not asleep...
I am going to get one as the master bed in my home, so a fold-away/sofa-type isn't on the horizon. I've seen some classy timber framed units which look good. I'll bear the matress issue in mind when I make the decision. My girlfriend likes the futon option, too.