First of all, I'm not here to rip Ben Affleck. The guy is a huge talent who found his groove and has proven himself as a top-flight director.
My first thought when I heard the news was that this is a risky move for Warner Brothers. Affleck is a fine actor in the right part. But, in my opinion, he doesn't submerge himself in a character the way a Christian Bale might and you need to go in deep to play a convincing Batman. Ben Affleck the actor has a sweet spot and I think Batman falls outside that spot the same way Daredevil did. The further you diverge from his natural persona, the tougher it is for him. I think Affleck as Batman is similar to a choice like Clooney as Batman. However, I don't think he'll kill the film.
Now, after a day of thinking about it, I actually think Affleck is the one making the bigger mistake. After a roller coaster career of triumphs and embarrassments, this guy has been cruising on all cylinders lately. I can't remember the last mistake he made. Gigli is a long-gone memory and no one thinks of him as some goofy guy that wrote Good Will Hunting and was the less successful half of "Bennifer." Ben Affleck is now firmly established as the serious and highly-esteemed, academy award nominated director. No one in recent memory has done a better job of rebranding themselves. In my opinion, he puts that all in jeopardy by putting on a cowl and cape at this point. And let's face it, no one has any idea how this Superman/Batman thing is going to play; not even Warner Bros. The thing could be good (...maybe) or it could be a dumpster fire.
I'll sum it up by saying, I don't think it was the most inspired choice by the WB. I don't think he needs it. I think it's a risk. But, I hope he knocks it out of the park like all his recent efforts.