There is an investigation going on at the moment, into whether Monster energy drinks caused the death of a girl who collapsed after drinking two of them - you can see the story here. Some other similar cases are being investigated too.
As the cases are under investigation, it will be some time before anyone can say for sure whether the drinks are really dangerous. The girl in question had a heart complaint, which will make it harder to determine exactly what caused her death. However, drinking that much caffeine isn't a good idea anyway - it's like having several cups of coffee all concentrated into one shot, and this is a lot for your heart to take. This is a problem with energy drinks generally, as well as the fact that they tend to have a lot of sugar or sweetener in them, which can undo a lot of the good that you get from doing exercise. Most reputable sports professionals will tell you that fresh, natural foods and drinks are much better for sustaining energy than artificial boosts.