I personally believe its a brilliant choice, we all know Affleck has made films like Gili, Pearl Harbour, but everyone forgets to mention how he was in Armageddon, Changing Lanes and The Sum Of All Fears. In the last few years he's turned his career around, Argo, The Town and The Company Men, from what I've seen personally or heard of others, he's been nothing but brilliant since he had his bad stint.
Now this is a risky statement, I understand that, but Daredevil, for me, was a good movie (the directors cut version). Mark Steven Johnson the director and screenplay writer for the film from my knowledge hasn't made many good movies. I believe Affleck gave that movie the only good aspect of it, and because of the character he played is the main reason he, to me, is perfect for Batman. The character of Matt Murdoch in that movie came across as a man, who's been through a lot, who knows the world can be an evil place, yet by some miracle, he has an ability he realises he can use to his advantage to made the city he lives in a better place, even if it's just for 1 person at a time at 1 night at a time. So he uses what he has to do that, all this whilst containing an everyday life, which also involves helping people, and I think that portrays well to batman, plus you can't deny he looks like a damn good Bruce Wayne.
Finally, I'll admit, Scott Adkins was my first choice for Batman.
This to me, looks like batman and Bruce Wayne. He has an unbelievable fighting talent, he's one of the best martial artists I've seen for a long time. He could make Batman look as badass as he could look, the character supposedly knows 120+ martial arts, I think he could pull the scenes off well. He was my first and last choice for him, until the rumours of a 40+ year old batman, then it changed. So for now, I'm happy with Affleck.