I decaffeinated accidentally one time (I hadn't realized that the 12 pack of Diet Coke I bought was caffeine free) and I thought something was seriously wrong with me. My head hurt so badly that I went to the doctor. I finally took a good close look at the cans and realized they were caffeine free.
My DH battles terrible acid reflux and one thing his doctor suggested was going caffeine free. He did quit cold turkey and never got sick at all - lucky duck
He does still drink a little bit of caffeine because he really loves iced tea and restaurants don't offer it decaffeinated.