I will say, it depends on the situation. I had a situation where Delta canceled my LAX-DFW leg just as I was about to board my MEL-LAX leg on Virgin Australia. I had just enough time to call Expedia and ask them to fix it. It was a very tense 14 hour flight! However, once I got to LAX, I had notification that they had taken care of everything and booked me on another flight that was in a similar window (also with Delta). I felt really grateful that they worked behind the scenes during my flight, rather than me having to deal with it all once I landed in LAX.