My work colleagues and I have. To summarise:
1. The Sunnis (including the moderates) mainly see Assad as the problem. They don't despute ISIS are dispacable and a problem. All Syrian rebel forces (including Al-Nusra) are fighting both Assad and ISIS. About 75% of the population are Sunni (incl Kurds). There are large numbers of Sufis fighting in the conflict as well as Salafists. Clearly the Salafist influence continues to grow but that is pretty understandable as these groups are the strongest militarily. There are still plenty of strong non-extremist groups about; the Southern Front who control much of Deraa south of Damascus are an example.
2. The Kurds are fighting courageously against ISIS but know full well they may well end up fighting Assad if he ever did regain control of what was Syria before civil war; they also are aware Turkey won't allow an autonomous Kurdish state to grow. The Kurds are largely Sunni.
3. The Christians, Yazidis, Druze et al are stuck between a rock and a hard place. They despise ISIS and all that groups stands for. They feel safer in regime-controlled territory. However, Christians in Syria have seen the brutality of Assad first hand the way with the bombing of places like Homs and Aleppo. About 10% of the population.
4. The Shia (incl Alawites) largely support Assad. They account for 15% of the population.
Clearly that is a broad brush summary. But it also highlights that the conflict is not simply black and white you like to state. Syria is an absolute cluster; it is incredibly complex. You have extremist and non-extremist elements in the north fighting with each other in north against Assad and ISIS. You have a dominate ISIS controlling the east, Assad controlling the west and former FSA controlling much of the South. They are all fighting each other.
Russian involvement is not going to solve the situation. Western backing of Assad won't solve the problem if they go down that route. The Sunnis hate him; go speak to someone who has fled Aleppo, Homs or non-regime controlled Damascus for example.
I don't offer any solutions or state remove Assad. I have been pretty clear on here that I don't think there is a solution. Syria and even Iraq don't now exist. We must defeat ISIS as they are barbaric but if you think siding with Assad will allow us to do that without alienating 75% of the Syrian population along with many Iraqis, you are sadly mistaken.
Sorry but I don't accept your claim that 75% of Syria's population oppose Assad. If that was truly the case he would have been defeated long ago. The same was said about gaddafi and Hussein but in reality these brutal dictators have millions of people that support them be it on ethnic, sectarian or tribal grounds whether you and I like it or not. We(the west) should work with Russia to defeat Isis and al Qaeda in Syria then help with the reconciliation process of all Syrians and let them decide their future rather than London, Washington, Paris and Moscow.