Trying to use multiple monitors on an Nvidia card and nothing but pain. They work just fine under Windows.
http://brainacle.com/multiseating-with-kde-and-xbmc-like-a-b...
Unity had a few bugs which seems to be fixed in this release but nothing annoying enough for me to switch.
Ironically, I also ran two screens on an ATI card which I bought to support their open source efforts. The nVidia was much better!