WT777
CPDLC is used in Australia, as GK mentioned above. It is currently used mainly in Oceanic areas, where VHF coverage is limited or non-existent.
The major advantages so far:
1. Eliminates the need for HF comms (and in Australia at least, these comms are done by a third party HF operator, not the ATC)
2. Both ATC and pilot have a text record of the communication.
3. Apart from 'free text/ free format' messages, most messages require some sort of logical response, and cannot be 'closed' until this has been done. For example, if an aircraft requests a level change, ATC must respond using a number of preformatted responses, ranging from 'unable' to 'climb/descend' to FLxxx'.
4. As most messages are pre-formatted, language difficulties largely disappear.
5. Most CPDLC equipped aircraft are also ADS equipped. Most routine position reports are done via ADS.
A limited trial of CPDLC in areas of VHF coverage is likely to take place later this year, and use of CPDLC is likely to spread to more and more of Australia.
The major disadvantages so far:
1. Because of the reduced voice communication, other pilots are less likely to be aware of traffic in their vicinity.
2. There have been instances (at least one) where the wrong aircraft has been given the instruction. In the case I am thinking about (and don't quote me on the facts) I believe that 'authority' over a CPDLC connection was not properly transferred between two centres. Some time later, the controller intended to give a level change to an aircraft with a similar callsign and inadvertently gave it to the aircraft a few hundred miles away in the next centre's airspace.
3. Pilots are still required to call on the appropriate VHF frequency, even if CPDLC is used.
Overall, it has been successful. It is a bit like the difference between e-mail and talking on the telephone.