I was able to work out pasting into tmux from the system buffer, but no solutions I've seen for copying out of tmux have ever worked for me. So yes, that's a pretty basic usability issue.
I have a plugin I use for that [1]. I enter copy mode, either by scrolling with the mouse or hitting Prefix + esc. I have the copy mode key bindings set up in vi mode (as is my shell), so I navigate around, hit V to enter visual mode, select some text and hit y. Now that text is in the system clipboard.
