How often did Entente units fight together as an integrated force?

[sheldrake]

Most of the time on the western front the forces of the Entente powers fought as separate national armies. But there were occasions when British French American and Italian units combined to work at a tactical or operational level. 

#1 How often did it happen?

#2 How well did it work?

#3 Did inter-operability get better during the course of the war?

#4 To what extent did the appointment of a supreme Allied Commander make a difference?

Any examples or observations?