Multi-Monitor
I currently use 4 monitors with Ubuntu 12.04. It's definitely changed a lot since 10.10 when I first starting using Ubuntu. Firstly, you (can) have the unity toolbar on all of you monitors, which I like a great deal. That way for me at least I don't have to move back 3 monitors to get to the toolbar, not a huge deal, but it's nice. I can't speak for those who have AMD/ATI gpu's but I have 2 NVIDIA 550 ti's and I have the the 4 monitors setup in xinerama so I can move programs/windows from monitor to monitor, and I have no issues thus far. The only problem with xinerama is compiz will not be fully functional as many of you probably already know. As for 2 monitors with compiz I do believe that twinview is the only answer (correct me if I'm wrong). However I was reading an artitcle a few weeks back that (forgot the link, sorry) canonical is working on a solution for the xinerama/compiz problem and have had some success but it may take til 14.04 for it to be implemented.
