It's not as good as native scrollback though. For example, by default, as soon as you select something in tmux, the selection goes away, and you have to hit a tmux-specific keybinding to paste it back into the terminal. That's
never what I want! If I'm selecting something it's probably because I'm going to copy it to my system keyboard. I think you can disable this part, except of course what you're left with at that point is a tmux selection, which is NOT a terminal selection, meaning you
still can't copy it to your system clipboard.
Also, scrolling is somewhat unreliable, although I still haven't figured out why.
In any case, I use tmux in some of my terminal tabs, and very frequently I have to hit ⌘R to disable terminal mouse support just so I can select & copy something without tmux interfering (I could also hold down the Fn key, except I use an external Das Keyboard, and the Das Keyboard folks still haven't figured out that their Fn key should actually behave like Apple's Fn key and let the system know when it's pressed by itself, as opposed to what it does now which is simply modifying the keypress events for other keys without sending any independent event for the Fn key).