Currently working with two monitors and but I definitely need a third one. At any point in time, I have at least four different browsers open, two or three adobe apps, a few text editors, communications windows, terminals and so on.

I feel I am more productive, because it's all about viewing space. If I have to alt-tab, or exposé between windows back and forth, that's a time waste.

If you're going for maximum productivity, I don't think you can really achieve it on a laptop, unless you're in and out of the office on a daily basis. I'd say if you're looking to get the most work done, use multiple monitors.