Batman is - to me - one of those dual roles. You have the Batman side -the super-hero, dark, man with a purpose fighting evil that skews towards the "crazy" end and then you have the Bruce Wayne side - man of the world, business mogul, charity ball attending, life of the party, heart-throb. It's a little like James Bond - who has to be equal parts action hero and subtle spy.
Actors cast for these parts tend to favor one side and fall down on the other. Sticking with Batman. Michael Keaton was probably the best at both sides, for all he was slightly in the middle with both of them equally at least. George Clooney was a really good Bruce Wayne, but a rather awful Batman. Val Kilmer was not really enough on the Bruce Wayne front, but his Batman was... let's call it passible. Christian Bale... I really did not think he managed Bruce Wayne at all - not the traditional Bruce at least, more of a playboy young-Bruce maybe - it was a little too Dawson's Creek for me with the Rachel thing, honestly. He was good at the Batman side (minus the increasingly intense voice-thing).
I think Ben Affleck has a shot at being good on both sides, but, like Keaton, I think he's going to end up in the middle on each. Which is, for me at least, better than being good at one and not at all good at the other.
It's a hard role to pull off, given the extremes of the character.