He wasn't one of the names floated to play Batman. Some wanted him to direct the next Batman solo movie. But given his comments about not liking playing Daredevil I guess people just assumed he wouldn't be up to playing another superhero. But Batman isn't just another superhero. I'm not sure how you turn this role down. I'm not sure if there's another job in the world I'd rather have than playing Batman in a movie, even being president. So I doubt it was a tough decision for Affleck.
I'm optimistic about the choice. He's got the look. He's 6'4 with a solid build. He's 41, which is only two years older than Christian Bale. And he's a good looking guy, not that Bruce Wayne necessarily has to be good looking. I'm not sure he's as versatile an actor as Christian Bale. How many are? But I think Affleck has the ability to be a good Batman. Most of his roles lately have been more serious than those of his early career. He spent much of Argo (which was good but a bit overrated) brooding, which is an important aspect of Bruce/Batman. And he spent much of The Town being intense without overdoing it, which is another thing Batman is asked to do. The key will be switching from the brooding, rage filled Batman and non-public Bruce to the billionaire playboy persona of the in-public Bruce. Based on his Kevin Smith movies I think he can bring the charisma when he needs to.
So again, I'm optimistic. But I'm not going to make any predictions or give a more definitive opinion because we just can't know how it will come out. Everyone loves George Clooney. But Batman and Robin was terrible. I'm not sure Christian Bale could have done much more than Clooney with that script. The script is the bigger question than Affleck's ability. If the role is written well and the plot is intriguing, I'm sure Affleck will be fine. If not, he might have trouble pulling it off. Making movies is a team effort. Hopefully it all comes together to make a great movie. Congrats to Ben Affleck and good luck.