I think Gambon will do an excellent Dumbledore. I liked Harris but he was... well too calm. Dumbledore is a character that despite his age has a lot of energy and everyone who sees it senses he is a very powerfull person. I didnt feel that way about Harris. In movie 2 he couldnt even walk, he was very ill. Gambon will give Dumbledore that inner life stregth the character has, and i cant wait to see him in movie 5 (if there is) fighting Voldemort.