Ben Affleck as Batman sounds like the kinda thing a fanboy would picture in his worst nightmares.
It´s not for nothing this casting has caused such a crapstorm on the internet. Comic book fans in general flat out hated him as Daredevil and now he´s gone straight for the Big Kahuna, the frikking dark knight, arguably the most popular superhero of all time. For many, much of his initial stardom came in the shadow of his buddies Matt Damon and, to a lesser extent, Kevin Smith. His biggest hit was Armaggeddon, which had little or nothing to do with him. For years, many movie buffs have considered him a bad or mediocre actor. Then he had a major career meltdown, from which it seemed impossible to recover.
I myself am among those who´ve never liked him as an actor. I´ve felt like he had a tendency to always play the exact same character, and I never cared much for that character. Specifically, I don´t really see anything particularly Batman-esque about him, not his face, not his voice, not his acting style.
All of this said, there also was a crapstorm when Michael Keaton was cast as Batman - they didn´t have the internet back then, but it was so big it ended up on the pages of the frikking Wall Street Journal - and to this day, for many fans, he´s still the best Batman. There also was a crapstorm when Heath Ledger was cast as The Joker - and this time it was all over the internet - and he´s become iconic in the role.
Also with all of this said, Affleck has had a very impressive comeback in recent years. His work as a filmmaker and actor in The Town and Argo has brought his credibility to a whole new level. One could picture this being a huge opportunity for him to redeem himself for DD, to prove once and for all that the Affleck so many used to despise is dead and buried and a true artist in every sense has emerged from the ashes.
So, against all my instincts tell me right now, I´ll take a "wait and see" attitude. I´ve been pleasantly surprised before, and loved it. One thing is for sure: there´s no middle ground for this. This casting will either prove to be a left field masterstroke or a disaster of such proportions that will make DD and Gigli look like mere hiccups.