The productivity you get from a second display is phenomenal. Programming is a lot faster and especially if you work with graphics and have fireworks next to Sublime Text and a Chrome window open at the same time. The workflow is quicker and if you buy a calibrated screen to work with you'll see beautiful colors and be able to see more pixels.
Get a screen that can tilt, rotate and swivel and you'll be able to turn it to portrait mode so you get more vertical screen estate which may be too big for a 30" screen to even look at.
If I'm not using the big screen for work it's amazing to watch movies in or to use it for gaming, you'd need a high-end graphics card to go with that too if you want to play it at native resolution. If you have three screens you can even benefit from eyefinity[1] and paired with a surround sound system you'll get a gaming experience that is parallel to none.
Really, I'd recommend two screens if you can spare the money, but one is already a big boost in productivity, and the second boost is much smaller.
[1]: http://www.amd.com/us/products/technologies/amd-eyefinity-te...
I used to be a big proponent of "more screens = more efficiency." There was a time when I was developing/running/debugging on a rather large network of in situ oceanographic sensors when I had four monitors arranged in a 2x2 grid.
Since then I was forced into working on some small, low-res displays in some cramped/uncomfortable environments. There's nothing like being afraid of heights and debugging code at the top of a swaying hydraulic scaffolding to make you want lightweight, productive efficiency.
I migrated to using yakuake, and now iTerm2 in a quake-like setup, and I do most of my coding in vim. I set the terminal and vim to the solarized-dark color scheme, and set the background at 80% opacity. I find this allows me to focus on what I'm doing but still "peer" through to see reference material behind the terminal. Further, because the terminal rolls down from the top with the press of a hot-key, I find that transitions between browsing/etc and writing code are very fluid and the burden of a mental context switch is minimized. This setup allows me to be incredibly productive on a 13" rMBP.
I recently had the opportunity to switch back to a multimonitor setup. I found that I spent too much time worrying about how things were tiled to make appropriate use of the real-estate, and began to wonder why I ever craved four monitors. In the end I just went back to my normal quake-like terminal/vim usage pattern as I feel this helps me focus better, and I really like the feeling of not being tethered to my desk/chair.
I use a standing desk, and find that I am far enough away from the monitors that head movement is not an issue; moving the pointer can be but you can increase the tracking speed setting to make that a non-issue.
What I am considering switching to is using the display attached to my laptop. Xcode would be on the laptop screen and the display would be used for showing reference material.
http://www.amazon.com/MeshWe-Bluefin-Seamlessly-metallic-sil...
I also use a different virtual desktop for each set of tasks. (email/hn, development, research, …)
I do add a second monitor when debugging full screen applications, but find I don't use it otherwise.
An important variable here: My eyes have reached an age where they are no longer variable focus devices. My computer glasses are set to a fixed distance, so the center of the screen is in focus, but by the time I pivot over to the edge of my monitor, the focus is not sharp anymore.
This provides a multitude of useful workspace configurations, especially if you use a read a lot of documentation on screen. A 1280x1440 window is sufficiently large for a full A4/letter page without needing to zoom or scroll.
I would be extremely reluctant to go back to a smaller display. 1920x1080 is neither fish nor fowl in my opinion, being too large for a single window in most applications but too small to usefully split.
I have to say, that I right now, I'm preferring my setup at home. Not only is the screen beautiful, but the increased resolution allows me to have 2 vertical buffers of code in Emacs with a browser next to it. I sit far enough back that I don't have to use constant neck movements, but then again, I only have 1 monitor.
At work, I use a standing desk, and have the monitors mounted in a half-hexagon around me, so it's easier to just do slight neck movements.
I split the screen space up into conveniently sized tiles most of the time. Most often using only half the monitor width at most (like two 1280x1600 monitors side by side, but no bezel line in the middle). But occasionally using the full screen real estate for one program is really nice.