I saw JL on Saturday. I wanted to love it, but I didn't. I enjoyed it, at times quite a bit. 5/10 for me.
The major problem for me was simple: Batman. In cosmic storylines (especially Darkseid related) the only way the Batman works is if he is a nearly perfect combo of the World's Greatest Detective and the World's Toughest Man. Otherwise, he is just a bag of flesh that would get liquified by the other players. Seeing him fumble and flail about, not even being able to find Cyborg was sad and stupid. He should have found them instantaneously, forced them to join, using intimidation, blackmail or whatever is necessary. They all hate but grudgingly admire Batman, giving them all a reason to band together to keep an eye on him, because he is so scarily ahead of them on everything. Batman's intellectual superiority becomes the one thing that softens the edges of this dark world, the humor and the pathos can all be generated easily by having a genius-level detective/strategist Batman (it's how Morrison was able to have a thrilling Darkseid arc in his 1997 JLA run).
So it is not about Affleck's delivering a weak, ineffectual, bumbling Batman, it is about the -script not providing the Batman this movie deserved.