I fall firmly in the "I'll give Affleck a chance with this" camp.
There are several things going for him:
1) His experience playing Daredevil. Now I'll completely agree that overall the Daredevil movie was very crappy. But if you forget about the story/plot/directing of that movie and just focus in on Affleck's individual acting/fighting ability he actually did a fairly decent job. As a long time comic geek I think he'll be able to avoid many of the problems that movie had and help make something better this time.
2) Affleck can act. Say what you will about silly movies like Gigli but Affleck himself is a pretty decent actor as long as he has a good movie to work with. Clearly he has name recognition and is seasoned enough that he'll probably serve in a pseudo-mentor role for the younger Cavill.
The only real problem I have with Affleck in this role is getting past the idea of seeing "Ben Affleck in a Batsuit". Henry Cavill had a fairly easy time becoming Superman because he was a relative unknown actor. As an established actor Affleck's going to have to work the extra mile to help convince the audience that he's actually Wayne/Batman and not just Affleck pretending to be Batman. Even with this "problem" I think he has a decent chance to pull it off.
Remember that most people hated the idea of Michael Keaton as Batman and Heath Ledger as The Joker. For this movie Affleck doesn't even need to be absolutely perfect because more than likely he'll be playing second-fiddle to Cavill's Superman.
Is this going to be the style of batman we had in the former movies and the animated series and not the one that was in the Dark Knight crap? Dark Knight was interesting and all but I liked the previous batman movies more. They were more true to the comic and less of a hollywood thing.
In the overall spectrum of Batman portrayals (with Adam West at one end and Christian Bale at the other) I suspect that Affleck's version will be a notch or two less "severe" than Bale's. He'll probably interject a bit of witty, dry humor (especially as Wayne) to counterbalance Cavill's slightly rigid/messianic version of Supes.
