I had ACC on a VW Phaeton and loved it. After about a month getting used to it I found I couldn't live without it so it's a shame it's not an option on the GTI. It involves using a radar-like sensor behind the radiator grill (or, in the case of the Phaeton, buried in the front bumper) which makes for an expensive repair if you have a front end shunt.
You can see Mercedes S Class or Volvo S80/V70's with them by spotting the dark coloured sensor behind the radiator grille.