Even though caffeine has been found to be one of the safer ergogenic aids, it is still banned by the National Collegiate Athletic Association (NCAA) when found in urine concentrations exceeding 15 µg/mL.
With energy drinks becoming more popular, the question now is how much of these performance improvements are due to the caffeine vs the other ingredients in the drink. Caffeine is thought to create an increase in plasma concentrations of catecholamines which in turn, lead to decreases in RPE (Rating of Perceived Exertion). Other research has indicated that it may influence adenosine receptors, thus stimulating dopaminergic activity which leads to an increase in wakefulness and a decrease in pain perception. Another study says that the taurine, ginseng, B-vitamins and carnitine found in energy drinks do not contribute to improved performance, but the carbohydrate in them combined with the caffeine does.
Bottom line? Caffeine can be beneficial to performance, especially when taken 60-90 minutes prior to exercise in a dosage of 3-6 mg/kg. Energy drinks do not help any more than consuming some carbs with the caffeine. So, coffee with your bagel or cereal might be best and less expensive.
Resources: Exercise Etc., Inc.