I've worked with single and dual monitor setups in different situations in the past, but it's been a while since I last had two. Right now, I'm using a single Dell 24" that seems enough to handle adobe and web developing programs (I got used to just alt-tabbing my way through them), but I'm wondering if I should grab one of the smaller spare monitors in the office and add it to my workspace.
Last time I had two, I only used the second one as support, for displaying complementary information (mostly thumbnails and previews). The main disadvantage for me was that both of them were inevitably positioned in a diagonal, something that I found quite annoying.
To those who use dual monitors, are there advantages regarding efficiency (I've freelanced for most of my life so my work process is kind of... personal) that I might be missing? Or is it just a question of preference?
I use three 17s at work and a single 22 at home (laptop is usually closed under the desk). I'm not sure what you're running, but I find that in most cases a bigger monitor with Spaces on the Mac is just as good as a multiple monitor setup. Your eye can really only look at one monitor anyways, and having Ctrl + left and right as options is only a touch slower than moving your eyes from screen to screen.
It is nice to have the real estate, though. Here are some situations where I find multiple monitors useful:
In all of my uses, though, I have a screen that's right in front of me. Let your secondary be your secondary and don't let yourself be looking at a crooked screen all day.
There's actual studies on this:
There's lots of reasons why having more screens can be a benefit to people. Brendan included several.
One not mentioned is that the monitors that you can change the orientation to vertical (portrait) are great for email or web research.
If you have an iPad handy, you can even use that as a second monitor:
UPDATE: FYI, I actually decided to try the iPad option last night. I could not get the Xdisplay option working (mentioned in the arstechnica article) with my MacBook, however, Air Display works surprisingly well. It's a tad laggy, so you're not going to be editing video on it, but for email, palettes, etc, its really nice. Big bonus is that a) it's portable and b) self powered, so makes it a perfect second monitor option for mobile workers.