Caffein: How bad is it?

I have seen alot of articles, mostly in fitness magazines, on both the benifits and the dangers of caffein. I consume alot of caffein(two or three 32oz monster energy drinks per day, the sugar free kind) and recently I have been wondering if I might be harming my body. I enjoy my energy drinks, and they seem to help my workouts, but If there are valid reasons why this could hurt my performance or health I would really like to know. Please share your opinions, especially if they are backed up by research or experiance. Thanks.