Exceptionally good appointment. Just because he made a hash of managing Blackburn doesn't mean he is not a world class coach and, as such, a brilliant replacement for McClaren.
I have my reservations. His leaving had no effect on Man Utd, and you could even say they improved after he left. He failed at Blackburn (yes he was the manager, but he was a complete and utter failure) His appointment as Leeds first team coach was around the time they started to go downhill. We'll just have to see how it goes - the Man Utd players certainly like him, so that will help a lot, and it could be that the Leeds and Man Utd points were just coincidence.
Well, not so much coincidence as just untrue. His leaving Manchester United had no effect because Steve McClaren was immediately installed as his successor. For a view of what happens when United lose their top coach, reference last season. In the same way that Kidd is a far better coach than a manager, Fergie is a no kind of coach at all - he is all manager. Equally, it is no coincidence that United have begun to improve again since that Portugese wallah was appointed in the summer. Everything I have read about his time at Leeds suggests he has been a very big success there.