I switched to tmux mostly just to get easy vertical splits. The RPM based distros (fedora, centos) didn't have a patched version of screen so it was just easier to deal with. Also mouse control so you can easily drag around splits. You can also check out byobu which adds some really nice features on top of tmux.
A lot of people have also ditched irssi for weechat but I just haven't invested the energy because irssi works fine for me.