For me it's just very mild (as in I barely notice) headaches and a vague under-the-weather feeling; to others I probably seem a bit grumpy. After a day or two of abstaining, the symptoms disappear.
Sure, it's vaguely annoying, but nothing more. When I travel or something and end up not drinking coffee for a while, it's simply not an issue.
Recently, I had some stomach troubles and doctor told me to cutback or completely stop drinking coffee to help stomach heal.
I decided to go cold turkey. Figured it would speed up recovery. A bad mistake. Headaches and irritation was easy to control and disappeared within couple of days. But my body is so dependent on caffeine. I had constipation, I could not motivate myself to go workout in evenings, I felt the world was moving at different pace than me. I just felt all sorts of wrong and I keep on thinking it was just a pace it pass soon.
Finally, I could not take it. Went back to drinking as much coffee as I needand life is great now.
After only a few months I noticed my vision was getting blurry, lacking focus, and there was some visual swirling as if I was on LSD. Fast-forward another 9 months and colors were muted, all emotions were numbed, I was insanely irritable, and I felt completely distant from the world. I felt so numb to life that I was very close to seeking therapy due to increased thoughts of suicide. On weekends I never had caffeine, but one day I decided to try some tea. That little amount of caffeine was enough to kick me out of the withdrawals and and make me realize it was the caffeine that was causing it.
It's been two months since I last had caffeine and I'm still going through withdrawals. For a while I had to take ibuprofen because the headaches were so severe, but now it's just a slight head pressure. Colors are popping, sunlight is gorgeous, I can focus on words in a sentence without my vision darting around. It's better, but I know there's still another month or two to fully reset. I'm getting back to feeling like I did in grade school though which is amazing to me. I've always drank large amounts of soda since middle school/high school, so I've likely always been impacted by caffeine like this at a smaller scale.
I had actually tried stopping early on, but was sticking with decaf and only for two weeks which wasn't long enough to notice anything.
For some added info, I have the A;C form of http://www.snpedia.com/index.php/Rs762551 which supposedly makes me metabolize caffeine more slowly.
I take two weeks off from drinking coffee each year (not because of the coffee, it just works out that way).
I had horrible withdrawal the first year, and switched from 3ish cups of American style coffee to 3ish shots of espresso with water or milk. Same amount of liquid, but considerably less caffeine. I haven't had withdrawal problems since.