They can do a lot of damage in the region, they already seem to be. Born out of extremity they practice extremism. But they are small fries on the world stage. I don't like how the media upscales this stuff as if it is 'global' threat, and I don't appreciate fear among Westerners of these guys. Many Iraqis are feeling the ill effects of this whole atrocious conflict, and I feel compassion for their plight. In a local geopolitical context, Isis could play a fairly big role. As a 'unifying' evil in the region, I think are pretty much Al Qaeda 2.0. Not a threat to Western civilisation or dominance, but they could be used as a further justification for incursion in certain regions.
It seems Isis has some momentum at the moment. I don't doubt they are cocky. They haven't secured their territory yet though, not by a long shot. As I said before, i think this is an example of extreme circumstances giving birth and momentum to perversion. It is crazy how fucked up the fallout of our invasion has been. These guys are probably even worse than Saddam...