While interesting as a case study, ISIS is ultimately nothing, a flash-in-the-pan of opportunism that has no real staying power.
The term "extremist fundamentalist" might be pretty trite-sounding after all this time, but that's a perfectly accurate term for these guys. They're o out-there that they were disowned by Al-Qaeda. AQ, while certainly extremist in its way. envisions a world where it and its affiliates rule over the assorted sects and groups in the middle east with an iron fist. ISIS, on the other hand, envisions a middle east where the only people left alive are sunni salafists, and this al-Baghdadi dude ruling over those. That is obviously untenable and unachievable, especially given where the group is operating.
And with fundamentalists... the whole idea of fundamentalism, of any flavor is "I'm more 'pure' than the guy next to me." And hte more hardcore the fundamentalism ,the bigger the divisions caused by even minor differences of opinion. Let ISIS squat over its territorial gains for three months and they'll basically dissolve into about thirteen different armed gangs intent on murdering each other.
And third, those territorial gains are pure opportunism, rather than an indicator of effectiveness. Syria's forces are naturally focused on the west of the nation which is where most of the activity is, where the important cities are, etc. Eastern Syria's pretty vacant for the most part. Same with northern / Western Iraq, it's not exactly heavily populated and isn't the first concern of the Iraqi government. Also, as we've seen, the Iraqi military is... less than effective. However this is not always going to be the case; Syria is very handily crushing and subsuming the insurgents, and will soon turn its attention West. iraq is getting bolstered by Iran, which has a pretty capable military. To the south, Saudi Arabia has moved its forces to the Iraq border, just in case - and movement against Saudi Arabia is very likely to bring a US response beyond "military advisers."
What we're likely to see is either ISIS self-destructs, or a joint operation by the Arab league states plus Iran and maybe Turkey moves to crush them. either way ISIS and its "caliphate" is not long for the world.
However the repercussions are going to be interesting. The weakness of iraq's government and military has been very painfully demonstrated; in the northern portion of hte country, Kurds are calling for an independence referendum, which may lead to similar moves by Kurds in Syriua, Turkey, and Iran... which could lead to more fighting, obviously. Southern Iraq is further split by this mess, its Shia majority being violently targeted by ISIS, which has the appearance of support from the Sunni community (even if it's not actually supported by them). It's anyone's guess whether Iraq remains "Iraq" or is split into two or three states.
Syria and Iran are going to come out of this with very solid gains; Assad in Damascus will have affirmed his authority over Syria and achieved a place of political prominence that may lead to renewed diplomatic pressure on Israel with regard to GOlan - it's unlikley to become military, as Assad's been very adamant in the past about that being a diplomatic issue, and Syria's military will be depleted at this point. Iran however is not likely to take many losses, and will reaffirm its regional power and cultural solidification among the Shia of the Persian gulf area - Iraq, Bahrain, minorities in Saudi Arabia, the UAE, etc.