I'm not a big coffee drinker, but will indulge when I need a pick-me-up once in a blue moon. For me, the effects are short-lived - I'll be flying the first half hour and then crash and burn the next.
I know some people who are SO addicted to coffee - that they literally can't function without it (interestingly, more women than men).
These people will actually go through withdrawl symptoms if they don't get their caffeine fix - these usually manifest themselves as headaches and irritability.
Another thing - and sorry if this is a bit crude - but many of these caffeine addicts can't have a bowel movement without their morning coffee.
When you get this dependent on a substance, it kinda makes you wonder.
BTW, I prefer the taste of Dunkin' Donuts but the stronger-brewed Starbucks will wake me up (and deplete my wallet) a bit faster.