As one who uses datalink every working day I can say that it is a useful system but no way can it be relied upon or used as some previous posters have suggested.
Sometimes messages aren't received other times they don't get there in time. The system allows up to 2 minutes before it times out. This is ok over the atlantic or other great expanses of airspace where there is nothing in the way, but in central Europe 2 minutes is a very long time. Especially when the pilot has to read, interpret and react! Using the old voice communication method takes seconds - I don't think many pilots would be happy stuck at Fl250 requesting Fl390 because we couldn't guarantee they would get the message in time
Datalink is perfect for frequency and squawk changes and issuing direct routes but for separation it is not good enough
I heard a good analogy - Datalink is like text messaging, fine for arranging to meet your mates down the pub later on (send an SMS) But if you needed an ambulance you would make a phone call ( voice communication) not a text
