We replaced our replacement windows about six months ago. The ones we had were horrible. The heavy glass had caused the vinyl to sag so badly that some of them could no longer be closed or locked. We ordered new windows through a contractor we trust, who explained and demonstrated exactly why we won't have the same problems with the new windows. Aesthetically, the new windows, though similar in many ways to the old and having the same positive features, are designed much more attractively, with more glass and less vinyl for better viewing. Each window has two locks instead of one, and a grip that extends the entire width of the sash. They are sealed more effectively, to prevent air leaks. They also have all the correct features for energy efficiency. As for personal experience, I see no signs of sag and don't expect to. The sashes have a much lighter feel to them than the old windows, and it was that heavy weight of the glass in the frames that had caused the terrible sagging. These operate much more smoothly. So nice. Further, these windows have a limited lifetime warranty, so if the sashes do sag, we should be able to get help, as well as replacement parts.