I agree with whomever said Affleck could be either very good, or very bad in this. It could hinge on how he plays Bruce Wayne, and the dynamic between Wayne and his alter Bat-ego.
It reminds me of something someone (Roger Ebert?) said abotu why Harrison Ford is so good in action movies with lots of stuff going on around him. He is the fulcrum, not the lever. He is the stable piece amongst all the action, stunts, cgi, whatever is going on, never trying to upstage it all.
Well at least they just signed Cranston as Lex!
I've never seen Breaking Bad or anything else he has been in other than Seinfeld, so can only think of him as Tim Whatley.