If you don't have plans to see this movie, you can check the spoilers here and then come back.
All the pieces seemed to be there for this to be something special, and certainly it had great word of mouth, but for my money, things did not come together as they should have and I found myself quite let down. It was not so much a bad movie as a slow, plodding an uneven one, and there was some pretty terrible dialogue and painfully wooden monologues. I'd say Ben Affleck actually did a commendable job directing, as the technical aspects of the film were pretty top notch, but his acting performance was disjointed and all over the place, not really anchoring the story at all. I lay a lot of the blame for both the movie and Affleck's specific woes on Rebecca Hall being the female lead; I've yet to really enjoy her in anything I've seen her in and this was no different. I think her character is supposed to be naive or shellshocked, but she plays her just dumb, and the script does her no favors; her chemistry with Affleck is flat as I didn't buy their romance and the scenes they shared either dragged or made me groan. Jeremy Renner is the saving grace of the piece, bringing maniacal intensity and steely sadness to a part that could have been one-dimensional as well as coaxing Affleck's best stuff out of him and making me mourn the movie that could have been had it focused more on these two. Jon Hamm's smugness serves him well in some spots, not so much in others. Blake Lively is barely in the movie, but when she is she vacillates between sexy scene-stealing and awful parody. More or less each of the actors--with Renner and Hall being the exceptions at opposite extremes--did some pretty impressive stuff on occasion that was drowned out by blandness or awkward turns the rest of the time, which sums up the entire film more or less. Still, there was an awesome car chase and some good shootout scenes, so it wasn't a total waste outside of Jeremy Renner.