I'll say one thing for the Potter movies--they give Michael Gambon a steady income. Spotted him in several ads for Potter merchandise, and I hope he gets plenty of fat paychecks for each and every one. He's a great actor, and his performance in The Singing Detective is one of the greatest I've ever seen. If he can't get millions upon millions (not to mention every acting statuette from every award-giving body for the past few decades including the Oscars) out of doing that, he can at least make money off of this franchise.
I don't understand the lack of love for Gambon's Dumbledore. Harris has had his moments (This Sporting Life, Return of a Man Called Horse, Juggernaut, even as recently as Patriot Games and Eastwood's Unforgiven he's terrific), but when he was doing the Potter movies he was literally at the end of his tether. Gambon makes for a sly, cunning wizard; he gives you the sense that while he may represent authority and establishment, he'd much rather be with the students, having fun.
And then there's the Gambon in The Cook, the Thief, His Wife, and Her Lover--don't much like that movie, but he's terrifying there. Gambon would have made an incredible Voldewhastisname--as likely to plunge his wand right into Potter's eye socket as wave it.