2 Guns is a perfect example of what happens when two guys play off of each other really well. They make the sometimes witty, but mostly flat dialogue bounce around with impressive ease.
To be honest, this isn't a very good film. It is laden with stereotypical heroes and villains, it's got a silly plot and some ridiculous action. What it does do, with great skill, is just roll with it and make the most of it. A lot of that has to do with the two leads. I don't think it's an unfair assumption to make that this film would have been far worse without them.
It is perfect popcorn entertainment, which provides instant fun, but which is also forgotten the second it is over.