I definitely would never have thought of Affleck as Batman, but I am willing to give him the benefit of the doubt.
I think that is why I am so shocked. He wasn't even on the radar. But the more I think about it the more I do like the idea. I think he can really pull it off.
I am more concerned about the script than who they cast as Batman. Look at Clooney in Batman and Robin. While I may enjoy the movie as an homage to the campy 60s show, Clooney did not do a bad job acting as either Wayne or Batman. The problem was people did not want a campy Batman movie, or at least the level of camp that was in that movie. So if Clooney had been Batman in a less campy movie I think people would not be putting the blame for that movie on his shoulders.
If they set the tone of this movie like they did with Iron Man then I think Afflack will do just fine.