Is ADS-C used in practice?
ADS-C is used in oceanic environments, for example over the Atlantic and the Pacific.

FL350 to FL390 on the North Atlantic Track System is restricted to ADS-C equipped aircraft. Non ADS-C aircraft have to fly either below or above those levels (crossing through those levels is permitted) 

The text below is a part of today's Norht Atantic Track message issued by Shanwick Center (EGGX): 


  222021 EGGXZOZX
  
  (NAT-2/2 TRACKS FLS 310/390 INCLUSIVE
  
  MAR 23/1130Z TO MAR 23/1900Z
  
  PART TWO OF TWO PARTS-
  
  E MALOT 53/20 53/30 53/40 53/50 RIKAL
  
  EAST LVLS NIL
  
  WEST LVLS 310 320 330 340 350 360 370 380 390
  
  EUR RTS WEST NIL
  
  NAR NIL-
  
  F LIMRI 52/20 52/30 52/40 52/50 TUDEP
  
  EAST LVLS NIL
  
  WEST LVLS 310 320 330 340 350 360 370 380 390
  
  EUR RTS WEST NIL
  
  NAR NIL-
  
  REMARKS.
  
  
  TMI IS 082 AND OPERATORS ARE REMINDED TO INCLUDE THE
  
  
  TMI NUMBER AS PART OF THE OCEANIC CLEARANCE READ BACK.
  
  2. ADS-C AND CPDLC MANDATED OTS ARE AS FOLLOWS
  
  TRACK A 350 360 370 380 390
  
  TRACK B 350 360 370 380 390
  
  TRACK C 350 360 370 380 390
  
  TRACK D 350 360 370 380 390
  
  TRACK E 350 360 370 380 390
  
  TRACK F 350 360 370 380 390
  
  END OF ADS-C AND CPDLC MANDATED OTS