He was pretty bad. Saying that he was bad doesn't mean Harris was much better. What makes Gambon bad was that he got the opportunity to play the better, richer and more interesting Dumbledore in the better half of the series, but he stripped him of all his warmth. Dumbledore may be manipulative and may have made some questionable moral choices, but he is a man a lot of people in the HP universe willingly followed because he had an ability to empathise and make them feel he cared. And he often did care. Gambon's Dumbledore inspired none of that. Irony is that I think he got off to a very good start in Azkaban -- he was energetic, quirky and funny -- but then derailed totally from Goblet. He was fine again for parts of HBP and Hallows, but he remained wildly inconsistent even within movies, in my opinion.

Harris was pretty good in the two movies he did, but I am not sure he would have inspired the awe Dumbledore needed to in the later films. He was simply too frail of health. But he certainly had a lot of the warmth missing in Gambon, but didn't have the physicality Dumbledore needed in the later movies.

I have to say I do dislike the OOTP duel. Dumbledore looks really weak and scared in it while Fiennes just hams it up to another level. It is fine on a technical level, but it doesn't do Dumbledore's character justice.

Dumbledore was an almost impossible character to do well on the big screen when the script writers were going to strip him of his nuance anyway. Movie Dumbledore is a fiasco because the actor playing him didn't appear to care about consistency and the movie makers also wrote and directed him poorly.