Individual responses to caffeine vary, and these drinks should be treated carefully because of how powerful they are. Energy drinks' stimulating properties can boost the heart rate and blood pressure (sometimes to the point of palpitations), dehydrate the body, and, like other stimulants, prevent sleep.
Energy drinks should not be used while exercising as the combination of fluid loss from sweating and the diuretic quality of the caffeine can leave someone severely dehydrated.
When used occasionally, energy drinks are not necessarily bad for you, but they shouldn't be seen as "natural alternatives" either. Some of the claims they make like "improved performance and concentration" can be misleading. They are marketed as dietary supplements, and the FDA does not approve or review the products before they are sold. Some energy drinks have no caffeine but instead use the stimulant guarana, which is the equivalent of caffeine. Others may say that 1 can is 2 servings so you have to calculate the correct amount of caffeine.
If you think of them as highly-caffeinated drinks, you'll have a more accurate picture of what they are and how they affect you. You wouldn't use Mountain Dew as a sports drink. And a drink like Red Bull and vodka is more like strong coffee and whisky than anything else.