I've had days without coffee. No problem. OTOH, the taste and drug of coffee, when the coffee is of good quality, is not something I would consider giving up. Those of you who dislike it can keep your views to yourself.
Chess and Coffee

Coffee sometimes upsets my stomach so I have to be very careful when/where I drink it . I more often get my caffeine from cold drinks like iced tea or coke .

Totally unscientific and anecdotal update: I'm drinking black tea now instead of coffee and playing better. Is there a correlation? Who knows

I wasn't asking you specifically But such a question isn't important enough for an expensive medical visit in the U.S....I'll have to stick to my unscientific, mad-scientist experimentation! Back on coffee now and playing fine so.....results are inconclusive!

yeah i've noticed this too. Coffee in general gives me brain fog, i play chess a little worse, read worse, am less put together when i speak etc. But coffee at work is so much better than no coffee so i drink it. If im off work for a few days ill only drink tea and my game improves.
Coffee actually helps me focus. For some adults (and some kids) with ADD, caffeine actually helps calm you down. I can drink 5 or 6 cups of coffee and, other than having to make trips to the bathroom, immediately go to sleep like a baby. So, "one size doesn't fit all" and I guess the secret is in knowing your own reactions to caffeine and choosing accordingly.