Web developer. I use three monitors.
It's absolutely essential. I have code open on one screen, the browser to show updates on another screen, and notes and other random stuff on the 3rd screen.
It's useful for lots of other stuff too. Doing taxes and business paperwork: notes on one screen (tiling windows to keep them organized), forms to fill out on another.
The effect on productivity is absolutely enormous. I could never go back to a single screen workflow.