While some heavy rock aficionados have bemoaned the current mainstream state of the genre in a market saturated with cookie-cutter pop music, the Rockstar Energy Drink Mayhem Festival is generating plenty of thunder. "We're playing between 10,000 and 25,000 people every single night who have their fists in the air and know every single word," says Zacky Vengeance, rhythm guitarist for co-headliners Avenged Sevenfold. "That's nothing to complain about."