Multi-monitor support in 12.04/gnome3 is unbelievably good. There are tons of horror stories, that I've lived and that I've read, regarding multi-monitor breaking jut when it's most needed (presentation at conferences, etc). This has happened to me in the past. Now, my system keeps track of my preferences per-configuration, so plugging in the monitor that I mirror display on automatically does mirroring, and plugging…
Multiple monitors need a good window manager. I sacked off my second monitor out of frustration in the past, I wanted shortcuts to move apps to the other screen, the ability to turn off one screen when not needed - I inevitably had to turn it back on, when I'd loose a window or dialogue somewhere. Full screen apps chose the wrong monitor, some windows sat between monitors etc. It was just a bind, and a distraction in the end. Virtual desktops were a better fit for me because of the inadequacies.