You are right, there are MANY claims that Energy drinks (Monster, Rockstar, Redbull, Redline, 5-hour energy, Amped, Ballz, ad infinitum) are bad for you.
I personally drink Monster Absolute Zero. I have never like coffee. I am 30 years old, and have been drinking energy drinks for 11 years (that's my qualifier). People will tell you the taurine can cause siezures (witch it can in high levels), and the caffeine content is 10 x that of coffee (which it is not). Monster (16oz) has about 160mg of Caffeine. Am 8oz cup of coffee can have between 150-300mg of caffeine, and nobody drinks 8oz. For me, it boils down to enjoyment, I don't enjoy coffee, I do enjoy Monster.
In terms of boosting metabolism, strictly speaking, caffeine alone can contribute to a boost in metabolism. If you are drinking a calorie-free energy drink, like Rockstar 0, etc., there is little nutrition, but the caffeine will boost your heart-rate for a period of time, thus causing you to increase your calorie burn... boost your metabolism however.. ehh, unlikely (could be true, but I can't find any studies)... again, really boils down to preference.