I've always thought of Millar as Ellis without the same breadth of imagination. That said, he's usually pretty entertaining. Ultimates was fun--although the first series lost points for the totally original concept of Nazi aliens (sigh), it's does a pretty good job of world building and the real point of the story is to showcase the assholes who comprise our Ultimate Universe Avengers, which is neat although not everyone's cup of tea.
Yet I can't help but wonder how much lower my estimation of the book would be sans the beautiful Bryan Hitch and Paul Neary artwork. Certainly I never read Ultimates vol. 3, before I ever even heard Loeb's writing was beyond terrible.
Civil War is okay, too, although it's pretty obvious whom Millar feels the "bad guys" are, even though they win. I wish Stark had just blasted Rogers' stupid, smug face off.
His Authority wasn't very entertaining, although again that may be because Frank Quitely draws like someone in diabetic shock and who has only seen one rather unattractive face in their entire life.