I must admit, that while I have no problem with the casting of Affleck, I don't really care for the idea of pairing off a Superman at or near the start of his career with a weary and tired Batman, who's been at his job for a long time.
We had a weary and tired Batman in The Dark Knight Rises and while I appreciate that this movie will doubtless handle him very differently, I would have preferred Batman and Superman to be at similar sorts of stages in this movie. Batman may have been doing it a little longer than Superman but I would have preferred to see him at, say, around the stage of his career that he was in The Dark Knight.
It makes me wonder how long they plan to keep this incarnation of Batman around for. Affleck is 41 but looks younger while Cavill is in his late 20s, playing his early 30s. Shouldn't they be played by contemporaries or at least shouldn't they be near-contemporaries in the movie (if Cavill can be 33, I would find Affleck believable as 35 or so).
I would've thought that the plan would be to use this movie to reboot and relaunch a series of solo Batman movies, without the need for an introduction or origin movie, but if Batman is meant to be tired and weary already, I wonder how much scope for solo movies there could be? Perhaps the end of this movie will see him reinvigorated and animated in his mission; I don't know.