I think as an actor he will do a great job. Basically all of his failures have been linked to generally botched films, which I don't think this one will be by any stretch of the imagination. Worst case scenario, it will be passable.
As far as his face and his frame go, he blows the previous Batman actors out of the water in terms of faithfulness to the source material.
I'm a little bit concerned with his voice, because the pitch of it is a bit high, so it could sound out of place when he's in the batsuit. Then again, they might modify it in post slightly, I just hope it won't end up as butchered as Bale's was.
One thing where I feel Affleck is a slight letdown is his shape. Now, I said that he has a fantastic frame, but as from the looks of the latest photos dating back to mid December, it's a pretty safe bet his shape will be far inferior to Cavil's. I know the guy is in his late 40s, but with a frame like that, I'd have expected a bit more. He's lacking in the legs, traps, and upper arms department right now.
I'm pretty sure he can can cut down to "The Town" body fat levels or lower until the point they film a shirtless scene, I just don't see how he could fix the aforementioned problem areas until then. Unfortunately they'll have to rely on the suit and camera magic to keep him comparable to Cavil.
I for one think a shirtless scene for Affleck is a must, moreso than it was for Cavil in MoS, since we're dealing with a more experienced Bruce, it's an opportunity to both show off his battle scars (kind of like what they do for Amell on Arrow) and his physical shape.
It would be a nice homage to this: