tmux is like that thing that you hear about quite often, that you think it's not that useful, but that, once you try, you realize why and where its fame comes from.
I adopted tmux about a month ago and I can't really complain much. It's just awesome. If you work a lot in the terminal (I do), I would highly recommend using it.
I use screen quite a lot -- and with xmonad as my main wm, I really don't need many features from screen -- I just have one screen per terminal window.
A friend of mine uses OS X -- and there tmux comes more to it's right as there's no (Free) way to get sane (for biased values of sane) window management under OS X.
[edit: It might be worth noting, that after running into a lot of problems with unicode input and urxvt, I switched to sakura[1] for my terminal -- and it's basically a lighter gnome terminal that's easy to set up without any chrome. Under debian I bind that to my x-terminal-emulator alternative, and I can ctrl-enter to open a new terminal via xmonad (xmonad needs a short config to use x-terminal-emulator though).
Well, if you use screen and it works for you, then it might be a good idea to keep using it. Why change something if it works? :D. Both tmux and screen are similar. By the way, that's the config I personally use and it works pretty well: https://github.com/tony/tmux-config.git.
I adopted tmux about a month ago and I can't really complain much. It's just awesome. If you work a lot in the terminal (I do), I would highly recommend using it.