Apparently they consist of a huge amount of sugar, which means you are getting a fair number of calories but no useful nutrients such as vitamins. The sugar can cause blood sugar problems in pre-diabetics and diabetics, and of course, can cause weight gain.
They also give you that boost of energy by containing caffeine. Caffeine is a diuretic so if you drink an energy drink you will lose water from your system. If you are outside in hot weather or working out heavily you can get dehydrated. Caffeine can also cause jitteriness and anxiety, and too much caffeine combined with alcohol can cause heart failure.
At one time Rockstar also had a lot of guarana in it, too, and still has a bit, as well as panax ginseng, gingko biloba, milk thistle extract and taurine. Guarana is another stimulant, one that is slightly different from caffeine, but with the same basic effect as caffeine at about the same dose level. It is possible to overdose on caffeine by drinking Rockstar.
In moderation Rockstar drinks are acceptable as an occasional part of your intake, but it isn’t a positive addition to your diet and certainly shouldn’t become a daily choice.