Functional and comfortable, so far.  I have had the sofa bed for a little over 3 months, have slept on it once when I first got it, and found it to be surprisingly comfortable.  With that being said, I have noticed the firmness breaking down by quite a bit and totally understand another person's complaint about the two sides being of different firmness.   At this point, the back has retained all of the original firmness while the seat has gotten much softer.  Since I've only slept on it once, I can't really speak to how this difference could affect using it that way, but it's definitely noticeable.  One of the things I really liked about it was how firm it was, so I'm a little sad that it's starting to get squishy.  I actually don't even sit on it that much.  If you're looking for something to keep around for a while that is going to get used a lot, you might want to rethink this option.