I was involved in the nootropics community for years, doing Gwen.net style double blinded assessments on various compounds. The biggest thing I've learned (aside from the fact that almost all nootropics are ineffective or only marginally effective, the marginally effective being the most popular: nicotine, caffeine, modafinil and amphetamines) is, if you're using performance enhancing drugs, you have to be honest with yourself about this and be really nerdy about it. Proprietary blends and energy drinks are just bad news. I would recommend taking caffeine pills instead of energy drinks. You can easily optimize your dose and you know what your taking. You can also get a years supply of caffeine for like 20 bucks in pill form. Being honest with yourself about the fact that you don't so much like the taste of coffee as the highly-addictive drug in it can save you a decent amount of money.

There's no evidence that any of the other compounds in energy drinks beside caffeine do anything for alertness, and caffeine is a very safe drug.