Dual 24" Cinema displays here and they have been fantastic for me over the past 4 years. Quite a nice size. I would find it VERY hard to go back to a single display.
When the new MP comes out, I will either add a 27" as a central display, or go with 2 x 27" displays.
I love my 2 x 1200p displays, but I find more and more that I could do with 3 screens. One for email/messaging/ftp, one for photoshop, one for browser/preview or coding.
But yeh - 2 displays far better than 1. Just get 2 x 24" or bigger.