For anyone still using gnu screen, tmux.
Screen is for the most part "good enough" that any replacement that aims to supplant it completely needs to either be vastly and obviously superior or enough of a drop in replacement that it doesn't require any effort to switch (I'm most definitively not investing time updating config files or changing key presses unless the improvements are dramatic).
As far as I can see, while tmux might be "better" in some nebulous way, the value proposition is way to weak to invest time in switching for me.
And I never type anything to connect to my screen sessions. How primitive ;)
I have shortcuts for the main machines I use that opens a terminal window, ssh's to it using my key, and attaches to my running screen or starts a fresh one if none is running (rarely - most of my screen sessions are 1+ years old)
My sessions on less important machines run in screen windows.
I switched to tmux because it is better maintained and it has better screen splitting functionality. I also mapped screens commands over to tmux, so the switch was basically painless. Took all of 15 minutes.
If you're cool with screen, you're cool with screen. No worries.
And I don't use the screen splitting at all - I use splitting a lot in emacs, but never in the terminal, so that's basically a non-feature for me.
I want to create, remove and switch between windows, and have a status bar showing their names. That is all.
I guess these are the reasons I've never seen the point in switching.
I REALLY like that the hardstatus line equivalent shows the command that's currently executing (think: bash, make, ./configure etc). That said, if you don't use the hardstatus line, I don't think there's much of a reason to switch unless you want multiple different size panes which screen doesn't really support.