May I make a intervention into this fascinating thread and offer my humble opinion based on my life long neurosis that comes in the form of Syria? (apologies to my lovely Syrian friends, I love your country and your culture, but I hate your regime).
I don't want to bring my background into all of this, but its hard not too, I'm half Lebanese and Half Iranian and of mixed Sunni-Shi'ite parentage (not uncommon combo in this region, despite the image of the region that is popular in the West). I have a certain perspective on the conflict which puts me at odds with some (but not all) my family on this.
On ISIS,
Let me be clear that ISIS are disgusting, horrible and I look forward to their demise. But we in the West (I live in the UK now), have a very unrealistic image of ISIS as a group. Its true that if you look at the map of ISIS land they control a territorial mass the size of Great Britain- but they are not that large a force. The highest estimate put their numbers at 10, 000, which would not be enough to rule over 22 million Syrians or 30 million Iraqis. If you look at the map closely, you will realize that the bulk of ISIS's Syrian territory is made up of desert and smaller habitations in Eastern Syria. The majority of the Syrian population live in the West of the country and outside of ISIS domains.
ISIS maintains hold over Eastern Syria because of three factors. Firstly, they have bought the loyalties of strong local tribes (without whom, they could not rule). Secondly, those who aren't in tribes chose to stay under ISIS's rule because ISIS have returned law and order (albeit their barbaric form of it) to previously unstable areas. ISIS have rejuvenated the local economy and the locals are more afraid of the Assad regime and so chose to stay under ISIS. For many, its simple, obey ISIS and live and ISIS set very clear rules of what they expect. Thirdly, the Assad regime does not bomb or attack ISIS areas and so there is relative peace in these areas.
This is because ISIS and Assad secretly cooperate with one another. ISIS started of as ISI in Iraq in 2006, they fought the Americans and Shi'ites and were based in Syria. The Syrian regime was afraid of American actions and losing influence over Iraq and thus allowed ISI to have basis in Syria. More recently, ISIS and Assad collaborate on a number of fronts, including exchanging oil and energy between the two. By enlarge Assad has attacked most of the Syrian opposition who are also opposed to ISIS, and left ISIS alone, which enabled them to grow.
In Iraq, ISIS success can be attributed to two things. The rampant sectarianism of the Central Iraqi State, which lead to a non-sectarian but largely Sunni uprising (which was peaceful until the Iraqi state used excessive force to crackdown upon), but was converted into civil conflict. ISIS came in and promised to protect these Sunnis and cut an alliance with ex-Ba'athists who were members of Saddam Hussein's military and security forces. These guys revolutionized ISIS.
In short, only a mass Sunni uprising can finish ISIS off, but they won't do so, so long as Assad is still in power. But ISIS cannot take nor hold Western Syria, it lacks both the manpower and resources to do so. In otherwords, ISIS do not pose the threat to a post-Assad Syria that many outside the country think they do.
On Assad,
I agree, oust him. This was all his doing and nobody else can be blamed for it.