I was drinking 4-5 cups of coffee per day, about 10 years ago. I quit cold turkey. It was like having the flu for a week. I was miserable, headache-y, and felt like puking most of the time.
After that week, I felt better, but for about 6 months afterwards I had a really difficult time getting "in the zone." I was a computer programmer, and caffeine was my method for turning my brain on. My brain felt fuzzy, I couldn't figure out basic thought problems without a pen & paper, etc. After that 6 months, I felt normal and was able to concentrate.
I stayed off caffeine for a good solid 7 years or so, and then started drinking green tea. So far, I've been able to control it (limited to a few cups in the morning, and if I skip a day I don't get any headaches). If I were to quit coffee again, I wouldn't do it cold turkey; I would switch to something more mild, like tea, and wean myself off of it.