I've had three monitor set up for a long time. Older machine with two ATI 1300 & 1500 graphic cards. Newer machine with one ATI 5550. Normally have two landscape, one portrait, I keep the mixer on the portrait monitor. I personally don't like the Reason SSL mixer on the portrait monitor as it's too "squat" you can't resize, where as Reaper Imperial Theme is great on the portrait monitor.
With the old configuration two card set, I've used 4 monitors it's cool for auxiliary stuff (explorer, notepad, resource monitor, etc)