Have yet to hear this actually in use. I have the same thoughts of many others here and elsewhere, its all a bit pointless if a call is still to be made. I also think that the solution to this call still needing to be made is a bit of a joke:
"Frequency is overloaded, let's get the pilots to not call when they change frequency."
"They have to call, ICAO says so"
"Oh okay well then tell them to call when it's silent"
I mean you are going to get idiots regardless, but I would say that very very few intentionally talk over someone else, and the cause for crossed transmissions is normally because a few people were waiting for the silence.