I tried that once out of curiosity but it's a clunky solution. You can't really drag and drop properly and it's not possible to move apps/windows across. One solution I'm about to try out is using the large new iMac and two Thunderbolt displays for a total of three which would be optimal.

What's not optimal is the price of these things, however. I liked the fact that I could hook standard displays up to my Mac Pro, but the multi-Thunderbolt solution looks workable as long as one is willing to shell out a thousand USD/EUR per display.