Is ISIS Really a Threat to the United States?

U.S. officials are claiming that the terrorist group Islamic State in Iraq and Syria, or ISIS, is "now a credible alternative to al Qaeda." But what does that really mean in terms of ISIS' potential threat to the United States? After all, al Qaeda hasn't pulled off a successful attack in the States since 9/11, or indeed anywhere in the West since the London transportation bombings in 2005.