The quick answer is both. There is actually some talk now about regulating energy drinks as a medicine since the chemical concentration is so high. If you have a pre-existing heart condition or high blood preasure, STAY AWAY from energy drinks, as the high concentration of caffeine can easily lead to episodes of heart palpitations and even cardiac arrest. Also, if you are on medications you should really consult a doctor before drinking energy drinks. If your doctor has ever said anything to you about specifically limiting the amount of caffeine you intake, energy drinks will only make the problem worse.
That being said, if you are a healthy individual, eneergy drinks aren't that bad in moderation. That last part is the most important, especially because so many young people today drink them as if they were a soda or cup of coffee. One energy drink per day, while not fantastic for you, will not do any major lasting damage, no more than two or three cups of coffee would do.
In short, don't rely on energy drinks for the pick-me-up every day, but on occasion they can help. The main time I drink them is when I have 12+ hour days and need to be alert for all of it.
My local paper just had a good article on this, some of which is sumarized above, but if you want to check it out you can find it here: http://readingeagle.com/article.aspx?id=422963