When do pilots actually use ‘wilco’?
When instructed to report something.

ATC: Cimber 626 turn left heading 120, cleared ILS approach runway 09, report established
Pilot: Heading left 120, cleared ILS approach runway 09, WILCO, Cimber 626



ATC: OYABC airborne time 33, report passing Dalhem
Pilot; (Roger), WILCO, OYABC



ATC: Lufthansa 135, traffic is a Boeing 737 on 5 miles final for runway 09, report in sight
Pilot: WILCO, Lufthansa 135