WILCO
WILCO of course means: I understand and WILL COMPLY with your instructions/clearance etc.
WILCO has fallen on hard times in the USA as some think its corny (corny means old fashioned at least for this purpose).
HOWEVER I USE WILCO whenever I can. It is almost a perfect word and it can save valuble time on the radio, PROVIDED there is no confusion about altitudes headings or clearance limits.
REading back for the purpose of protecting yourself on someone's tape recorder is a good idea, HOWEVER if the controller doesn't HEARBACK ( basis of word is readback) what you say and only HEARS what he wants to hear, someone can still get into trouble.
and just for those who use the radio (RT for the british)
saying your call sign as a response really means nothing
saying your call sign with the word roger means you have heard what the other person has said.
saying your call sign with the word WILCO means you have heard/understand and will comply with what has been said.
and saying roger or wilco without your call sign just leads to potential problems.