I used to sleep on a futon. I found it to be comfortable, but then again I can sleep on just about surface with no problems. The main reason I had a futon was because it could be folded into a couch, (very convenient!) . I would suggest getting one with some sort of cover on it. On the one I had it didn't have a cover on it and whenever something was spilled on it it was a pain to clean it up. Mine had a metal frame and it did squeak a little and got a little annoying, especially when the lady friend came over.
I would also suggest getting an upgraded mattress. The one I had was a cheap one and it definitely got smashed down and rock hard in certain places, while other spots were still soft.