It's definitely not a complete alternative, but a surprising number of tmux users are only in it for the session persistence. We did a little internal (criminally underpowered and confounded) survey of some tmux users at google and found that half of them were only really using it for session persistence. I was really surprised by this and thought more people would be using all the slick tiling features that tmux has.
I think there is a whole group of vim + tmux people out there and their primary use case may not be session persistence, but to construct an IDE like environment, which is also my use case.
