I think ISIS could be a serious treat, not only to the western world, but also to all countries near Iraq and Syria. The situation is seriously escalating and the US, as one of the major world powers, should at least help to restore peace (and not by bombing the country, but by diplomatic solutions).
A state with an obliged religion, like a caliphate, is a direct treat to the whole world (not only the western world) and world peace.
It started with Syria and Iraq, but where will this end? Iran, Jordan, Israel, Libanon,...?
So in my opinion, US should definitely do something.