Dumbledore is a funny character and only an author who conviently forgets what she wrote in the previous books could come up with such a mess. Most people like to think Dumbledore is just a good man who makes a few mistakes, but his ability to always make mistakes in the most painful way possible for the good guys and the most helpful to the bad guys makes that rather supsect. Insane, addle minded or merely stupid could not produce that kind of consistent results.
My favorite rant about all this is Dumbledore's ability to find good people and shape their minds onto the right path. NO one under dumbledore's attention really achieved anything yet at no point did anyone tell him to retire or did the idea ever cross his mind he wasn't fit for the job anymore. Even at the height of Voldemorts second rise Dumbledore kept on being headmaster, like fighting ultimate evil was a weekend or summer time sport. That you could conviently ignore the trouble in the rest the world to shuffle papers around and eat lemon drops. If Voldemort wasn't so kind as to make all his plans go off at hogwarts and around Harry Dumbledore's defensive 'wait and see' aproach would of been even worse.
I think the biggest joke is many say Harry was being raised to be Dumbledore's successor as leader of the light, yet at no time did he get any real leadership training. Even the DA was all about what other people wanted, Hermonie talked him into it and all that. If Dumbles truely wanted a Leader for the next generation you have to wonder why Harry was bred to be such a mindless pawn, willing to follow bad orders to the death on just about any one elses command. Dumbledore, Fudge, Snape, even Ron's stupid suggestions like Duels at midknight or stealing a car are instantly agreed to.