DD has one and I find it pretty comfortable. We've haven't used the trundle part yet, but it does have a mattress on it. The frame is pretty supportive....plenty for a guest room! She jumps on hers all the time and I don't notice any sag or damage. The pop-up is easy to function and allows us to have room for cousins when they visit, but not have to have a big bed in DD's room. I know some people who when they use it as a king, put a king mattress pad on the two to keep them together better, or may put an "eggcrate" pad on it to disguise the crease in the middle. We haven't used ours as a king yet, so can't comment on that part. Personally, I think it's a great set-up, especially for a guest or kid's room.
As for the sag issue, I think that mostly depends on the quality of the mattress you purchase....