CPDLC
My opinion:
When I used it in Brisbane (FANS1/A) it was wonderful. Lots of different messages to use including differenct route clearances, questions - when can you accept FL370 for eg., freetext messages.
Particularly good for the big procedural sectors and no doubt invaluable for the oceanic sectors. Especially when used in conjunction with ADS-C reporting.
Working with it in upper airspace in Germany?
About as useful as tits on a bull.
Limited message set, in fact off the top of my head can only think of frequency changes and replying to level change requests initated by pilots. also DCT to a waypoint already contained in the flight plan route.
I am sure the message set will improve over time, however. Would be nice to be able to issue headings and route via waypoints not on original route.
Combine that with a lack of equipage amongst operators and sometimes quite a delayed response means it isn't that popular here in Germany.
Of particular note I notice a lot of long range aircraft logging on to Maastricht (who I think are FANS 1/A equipped) but thinking they have logged on to Rhein.