Strange,
everyone seems to agree not to readback. I was thought that ICAO required readback of headings and altitude. We don't have mode S at our facility, and then it really comes in handy when pilots read back the heading.
It happened to me only last weekthat a pilot did acknowledge his heading and still turned to a complete different one, so it's not a guarantee they will fly it
blueskiesup, you can just say "callsign, roger, left 123".. I think that covers it in a busy traffic environment.