Once, he has played Guitar Hero once. The reason for this is that he was once "addicted to video games." Hammett adds, "At one point I had to say 'enough.'" So that's what he said.
He has seen other people play it, though, giving him the confidence to state, "I'm pretty knowledgeable about it." I once saw brain surgery on the Discovery Channel, and now, I'm pretty knowledgeable about that. Not that Guitar Hero is brain surgery. It's not.