I have been using dual monitors for coming on eighteen months and love it. 

One problem with dual monitors when using OS X is the quality of the second monitor has to be very high for real Mac feel.  My primary monitor is the built in iMac monitor, and my original second monitor, which displays Windows just fine, looked like viewing a the Mac OS through a dirty window despite all my efforts to adjust settings and calibrations.  With the Apple Cinema Displays ridiculously over priced this was one of those small annoyances.

Recently I upgraded my second monitor to a Dell SP2009W (yeah, I've no idea how they picked such a cool name either).  This monitor is awesome.  Within about 10 minutes of messing with the settings, including one that put it in Mac mode, and calibrations I have two monitors that show off the beauty of the Mac OS to an equal degree.  I can now truly swap back and forth applications that I would n

I also got a 3M adjustable monitor arm that, apart from making the monitor look like it is suspended,  can flip the monitor between portrait and landscape modes.  And anything in between if you wish to code on angles.  I was excited to try coding in portrait mode as it seemed like it would show more code.  This is great for browsing a folder structure and working on one file but when a 20" widescreen monitor is turned to portrait it becomes very narrow.  The IDE features become very far apart from each other so for now I have gone back  to landscape mode.  Below are two photos with this setup.

Portrait view...look at all that code!

More "traditional" landscape.