I would have sexual intercourse with coffee it was socially acceptable. I love the stuff.
And yes, one coffee (either Starbucks or my special espresso stuff that i make in my fancy schmancy Italian perculator) is what i'll pretty much limit myself to. When i have that coffee my brain works half as fast and, unfortunately for those around me, i also have a tendency to verbalize every single thought that wisks through my head.
Sans coffee, i'm a vegetable. (For those interested, i think it was National Geographic last month that did a big expose on coffee and what areas of the brain it effects, etc. People really do think better after having had a coffee).
However, while the high i'm on usually supresses the dp, once the initial buzz wears off, i'm struck with slightly elevated anxiety. But if i go beyond my one a day limit, the anxiety wacks me in the face almost regardless of circumstances, and it is not at all pleasant.