I've tried using iTerm2's tmux support and found it to actually be less friendly than just using tmux directly. Definitely a YMMV thing, I suppose, but I was seriously baffled.
One thing I heard from several people in a previous discussion is that iTerm2 makes a thin outline around the terminal window and since they often have overlapping terminal windows (that are presumably opaque with black backgrounds), Terminal wasn't as usable. This has changed with Yosemite's Terminal, though.
Huh, I never noticed the very thin outline around Terminal windows in Yosemite. Neat. I've been using a slightly-transparent dark background with blur for years (which gets slightly more transparent for background windows). That always made it really easy to visually distinguish overlapping windows. But I can see the border being useful for people who have completely opaque windows (with black backgrounds).
One thing I heard from several people in a previous discussion is that iTerm2 makes a thin outline around the terminal window and since they often have overlapping terminal windows (that are presumably opaque with black backgrounds), Terminal wasn't as usable. This has changed with Yosemite's Terminal, though.