I personally think Affleck is a fine actor, he act well enough in the roles that he was in, but at the same time, he has been in some pretty terrible movies. Everyone raised the comparison on his role as Dare Devil, I thought the movie was just plain bad, no actor in the same exact role would save that movie in my opinion. So that's not entirely his fault. I think Affleck has the look as Bruce Wayne, he has the chin/jaw to be a good looking Batman, I think he will look better than Bale as a Batman, Bale has weak jaw line and lips, he was never a good Batman, Keaton's Batman looks so much better because of Keaton's lip and jaw. I think Affleck should probably look ok with a Batman mask on, hopefully without Bale's horrendous Batman voice.
Through out Affleck's career, doesn't matter what movie he was, he is always in the same role over and over. He is great in roles that was written FOR him, there's nothing wrong with that, you are not going to see him in a breakthrough role that make you say "wow, I didn't know he could do that!". He doesn't have a wide acting range, he is what he is. I don't ever remember him in anything intensive, even he was in bunch of physically or situation intensive movies, he has always been cool, calm and laid back. I think this aspect of him is perfect for Millionaire Bruce Wayne, but Bruce Wayne is deeper than that. This is a kid that saw his parents got murdered, suppressing his emotions all his life, there's that intensive underneath the playboy exterior. I don't believe Affleck can handle this kind of split personality.
I suspect whoever handling "Man of Steel" franchise is playing a publicity stunt game. Everything they do up to this point is to get everyone talking and continue to talk. They know what gives the fan a raise, and they give you just enough to get the fans' attention away from Marvel's movies. I didn't like Man of Steel, I didn't like they rush into Superman vs Batman, and I don't believe I'll like Affleck's Batman, I hope they proves me wrong though.