Probably the most common negative effect is the development of dependence on caffeine. Which is why even though there have been few studies that show about 2 cups of coffee a day has cardiovascular benefits (can't find the links, not even sure if the studies were done by proper medical research teams) I prefer to keep myself down to a max of one cup per day with no coffee every 2-3 days. Helped me alot to avoid getting dependent.
RE: The Effects of Caffeine on Sports Performance