My wife and I bought an expensive queen size futon for our guest bedroom. We anticipated my father-in-law and his wife visiting from out of town and sleeping over, so wanted it sized accordingly. We were hoping to maintain the room as an office the rest of the time, with the futon serving as a couch. The queen-sized futon seemed to be the best option.
As a bed, it's great. Very comfortable mattress. And my father-in-law (who is both very tall and fairly wide) fits comfortably with his wife as well. However, as a couch, it's almost unusable. Anyone who is not well over 6-feet tall sits with their feet dangling just over the edge, with their butts jammed way back into the "fold" of the mattress.
Also, even with large bedrooms in our house, the futon is so wide that there is room for only it plus one small night stand aside it.
...just a recommendation to check out the physical dimensions of the futon before buying, and make sure that you can deal with a piece of furniture that LARGE.
If it's going to be a bed almost all the time, maybe you're OK.