I'm glad that we'll all be able to appreciate another Harris movie. I think it was such a shock for all of us. He died so suddenly, and, because he played the seemingly-immortal Dumbledore, I think we all just took Harris to be seemingly-immortal as well. However, I think that the casting director had a really hard time finding another Dumbledore. And Gambon is just doing the best he can. The way I look at it, PoA, GoF, and OotP are all darker movies, and Gambon plays a darker Dumbledore, one that sort of fits into the story more. It would seem really odd to watch a dark HP movie, and have a light Dumbledore. But, Harris still captures the true nature of Dumbledore and he's the one and only Dumbledore.