What I've certainly seen and been briefed on is that....
You have to remember that operations in Syria against the Assad regime by both the Free Syrian Army and a conglomerate of ideologically motivated entities have been going on for almost two years. Those troops, those ISIL forces in Iraq, are battle-hardened. Add to that a number of senior former Iraqi military personnel, Baathist parties for example, disenfranchised from that. The assessment is that they're well led, well experienced, and very motivated. So they do represent a considerable threat to Iraq.