In my experience the effect is minimal and tends to be overshadowed by other, stronger effects. For example, at my last office job I had a laptop plus monitor setup at my desk, but unsurprisingly found that I was usually much more productive if I took my laptop to an empty conference room. Of course it's possible I'm just doing it wrong.

Right now I work at home and sometimes at coffee shops and I find the flexibility of choosing where I work (and in what posture) to be well worth the missing screen real estate. I am strongly considering the purchase of a new "Retina" Macbook Pro, but I don't anticipate huge productivity gains because of the extra pixels.