When I actually get around to writing, I always have a coffee on my desk.
It usually lasts about 25 minutes, even when I'm writing like a madman - and I normally write for an hour at a time. So I'll have a bottle of normal flavoured drink (no caffeine) on the desk as well for the rest of the hour.
I drink a lot of fluids when I'm writing... It's what I do when I'm thinking and not actually typing.
And then after the hour, I go pee, have a smoke, make another coffee, and write for another hour.
I'm not sure if the caffeine actually helps the writing itself or not... But I do know that it's few and far between that I can talk myself into working on a WIP if I don't have a coffee in front of me.
So I voted yes.
Oh, also - I've had so much caffeine in my life that I'm basically immune to it. It can help me wake up in the morning, and I love the taste of coffee (and Red Bull! But Red Bull's expensive...) so I keep drinking it. But I don't think it really affects me at all, not anymore.