For one, it is clear that ISIS alone surely did not capture Mossul and Tikrit within few days. Local Sunni tribal fighters as well as former Baath party officers had also their share, with the latter being behind a long term planning of the events.
At the same time nobody should have doubts that ISIS is the „muscle“ of what many disgruntled Iraqi Sunnis consider a „Sunni revolution“. Trying to play down ISIS´ role as the primary and most lethal fighting force would be a repetition of similar illusory claims regarding the Al-Nusra front in Syria, which western powers and Arab countries behind the Syrian opposition for a very long time tried to detract from.
In Syria the so called „Free Syrian Army“ had tens of thousands of fighters and was even „assisted“ by the already mentioned hardcore Salafi fighters of the Al-Nusra, but still ISIS managed to fight and rout these groups in eastern Syria and inflict heavy casualties on them elsewhere in that country.
There is little reason to assume that ISIS will „perform“ weaker in Iraq. Other Sunni groups including the more regionally interested tribal fighters as well as the more nationalist and secular minded former Baath party forces are welcome to cooperate with ISIS and contribute but ISIS will demand and enforce to have the final say and call the shots.
Should the Sunnis decide to „enjoy“ ISIS and tolerate the leadership of the „islamic state“, this will be the end of Iraq as a sovereign state as ISIS openly and proudly announces it´s firm will to fight the Shia majority of the country. ISIS is not a mere (and legitimate) resistance movement against an unjust, Shia led government but a vehemently sectarian, supremacist and violent movement, which considers all Shia as infidels who deserve death. But Iraq is not Pakistan where the Shia are basically defenseless. In Iraq the Shia make up a majority of 70-75% among the Arab population and have tens – if not hundreds – of thousands of battle-experienced fighters willing to die when existentially threatened.