Johnny Novgorod said:
Some actors, good or bad, I have a hard time forgetting who they are while they're acting. Me, I see Ben Affleck in a movie and I can't even process his character's name - he's just Ben Affleck playing out actions and emotions. Ben Affleck in WW2. Ben Affleck in that comic book movie. Ben Affleck in that reverse engineering movie. To see him put on the Batman costume would be... I dunno, look, there's Ben Affleck wearing the Batman costume.
Basically this (though in all fairness I haven't seen any of his latest work. The equivalent would be scratching your head over a Keanu Reeves casting with only Bill and Ted to go on).
That said, I say give the man a go. As others have said I doubt he'll have any problems as public-eye Bruce Wayne (he's got the smirk), and I'm curious to see if he can play private Bruce Wayne without coming across as the pretentious whiner he did circa his Kevin Smith movies. To be perfectly honest I don't think any actor to date has been a competent Batman, though Bale has probably come the closest (but that was largely thanks to the script).
In fact, the more I think about it the more I think Affleck is a good cast for the "rich douche that jeopardises his dynasty and its shareholders by putting all his energy into dressing up and being a thug because he's too immature to deal with his grief like an adult" angle of Batman. I can see the plot now: Batman is being an angsty douche, Superman steals the spotlight from him by being a more public-friendly hero, Bats gets jealous and tries to peg Supes as the bad guy so he can justify beating him up, Supes flexes his uber-morality and makes Bats see his "justice" is all about serving his own interests, Bats hangs up the cape and cowl and turns his energy towards running Wayne Enterprises, but during Wayne's absense the company had done some shonkey real estate deals with Lex Luthor, Supes tries to play the hero only to get himself in a pickle, Bats suits up one last time to kick Luthor in the nuts, Supes and Bats make out, Supes flies away to fight more crime while Bats returns to directing his empire. Oh, and there's dinosaurs somewhere.
Also, am I the only one that's ok with the Bat Voice? I mean, the last thing he wants is for someone to say "Hey, you kinda sound like that Wayne guy that's always on the tv..."