For 10.10 multi monitor support was rudimentary, as that was focused on netbooks -- if you were giving a presentation and you ran Unity we didn't want the top panel overlaid on your presentation.
For 11.04 and 11.10 the multimonitor support is much better. I'll talk about how my setup works with Nvidia, but I don't have the hardware to comment on ATI and Intel support so hopefully someone will post seperate answers for those.
For 12.04 and 12.10 there is a specification for how to improve multi monitor support in the desktop.
Each monitor gets a panel with the window controls, the application menu, and then the indicators. When the windows are maximized each window gets integrated into the top panel of each screen. The indicators are cloned so that you're never too far from the system menu.
When the windows are unmaximized the application menu for that app will be displayed on whichever panel the application is on so that you don't have the menu all the way on another screen:
Using the open source ATI driver with RandR (you setup from Monitor Preferences, see screenshot) then the same result is achieved as with Nvidia Twinview.
Here is a list of possible combinations of multi monitor setups and their results:
There are some bugs (other than the first, they are generally small aesthetic details):
- Bug #742544: For users with the secondary monitor on the LEFT of the main monitor, the launcher displays in the wrong place and similarly regarding [the main panel's position, Bug #757652]
- Bug #769458: Desktop background not correctly rendered on startup
- Bug #742020: Indicators being shown as selected on both screens
- Bug #743621: Occasional problems with applications that run in fullscreen
There's still the potential to improve support further, for example the suggestion in Bug #755357 to allow two global menus to be active at the same time.
See also What's the right terminology for Unity's UI elements?