Well, it depends how they play it. "Thor guy" as Mockney puts it works in the Marvel films because they're deliberately hammy and over-the-top. It's the same reason RDJ absolutely nails Tony Stark - they're meant to be a little bit silly.
The DC films, on the other hand, have been going for this "realism" angle that requires a totally different skillset. Bale was a good Batman because he just went around being introspective and angry all the time. The Superman film tried to keep the same tone whilst including aliens and spaceships and failed spectacularly because of it. Affleck might be able to pull this off -- he's actually a pretty decent actor -- but if he's just trying to be Bale Mk. II then he's going to fail. If the tone of the movie is a bit lighter, it might work.
I fear that the inevitable Justice League movie is going to try and maintain the gritty style used in the Dark Knight trilogy, though, which is surely not going to work. You can't be hard-hitting, serious and poignant whilst a bunch of men in tights fly around shooting lasers from their eyes.
