Affleck wouldn't be my first choice for Batman but it occurs to me; how could he screw up the role any worse than Christian Bale did? Certainly Ben will have no trouble matching the total lack of charisma Bale brought to the role and the ridiculous fake voice. The other thing is that the way Superman and Batman have been written in the Nolan movies, neither role requires much acting talent.
Ultimately, Affleck's grade in the role will be tied directly to the next movie's box office no matter what the fans think of his performance. Honestly, if you loved Nolan's Batman movies, I don't see how the selection of Affleck to play Batman can be that big a problem for you.