Incident Command System is used in NYC and it doesn't make anyone happy. But the way it works is as follows...

ICS is used during Multi Casualty Incident (MCI). Whatever MCI is can vary from time to time but usually for our purposes it is more than 5 patients. If the scene is fire related than things are pretty simple since only two agencies have to work together (medical and fire branch). Things get complicated when crime is involved and now you have another agency comming on scene that is totally ignorant of the other two. This is where arguments begin: since this is a crime scene than police should be in charge but there is also fire involved so fire dept should take over. In theory ICS works like a charm but in practice it is a big headache. Dont get me wrong it works better than nothing but before all the gears start turning you have this initial time when chaos rages. Every agency is on different frequencies and everyone wants to be a hero.

So in theory:
Four alarm fire happens. Fire company gets dispatched and few ambulances. Firemen fight fires and their Lt. is in charge until Capt arrives and sets up Incident Command Post. First on scene Lt usually will stick with the Capt since by now he is the most informed person on scene. First ambulance becomes medical traige and determines priority of transport for pts and supervises proper transfers. Ambulances that arrive later provide treatment. Police secures the scene. Guy in charge delegates Lts who take care of transport, communication, supplies and other important stuff. This should be set up asap but it may take few hours and until than everyone is running around like chickens with their heads cut off.

Now this is for the city. Probably the best ICS I've ever seen is at JFK airport where I moonlight. Why? Because Police, Fire Dept and Medical Agency belong to Port Authority Police Dept and they train and work together.

In conclusion: system works but it has a slow onset and that's what causes the biggest toll on patients, bystanders and rescue personel. If all people working together were willing to put their egos on the sidelines than everything would be fine. Press doesn't help either when they single out one agency to get a credit omiting others.