I rarely use expedite climb or descent. But in any case I do mean the best you can give me. And it is in your benefit because it is then always for seperation. However when a pilot reads back "Roger expediting" I still have no clue on his ROC or ROD as it is not clear at the time what the expedite really is that the aircraft can do or what the pilot is willing to do. So basically I still am not 100% sure this way if I can guarantee separation. Better would be ofcourse to give a specific rate, just a shame if it turned out the pilot could do better.
As well the perception of an expedite climb or descent from a pilot's point of view vary a lot. Sometimes when I give to the same company, same path, same aircraft type a ROD of 1500' or more one pilot will come back as if it would be a low ROD and another pilot will readback; Expediting!
In other words I try not to use it, but when I do I suggest you go down like a brick or climb like a rocket
