Absolutely gripping from beginning to end. And yet...on reflection once it was over, for the wrong reasons, at least in part.
Much of the tension seems to come from the immanent sensation throughout that something particularly memorable is about to be said or occur on screen and then it mostly doesn't, but such is the pace of the film that this disappointment never becomes a kind of reverse thrust.
The best bits of dialogue are really monologue. My favourite, the line about German tourists occurs quite early on. There's another about NOLA — 1000 restaurants but only one dish on the menu — actually made me think of Antigua.
But the later dialogue moments, especially the scene with "Q-Tip" flatter to deceive, and overall Fincher sometimes seems to be overlaying too many words on top of a fast-moving scene (for a change!).
Tilda Swinton's character is labelled 'The Expert', though we never quite see why. The titular character played by Fassbender certainly considers himself an expert too, but throughout the movie's action seems to make mistakes or take silly risks that even a rank amateur would spot, especially one that kept reciting "anticipate, don't improvise" to himself.
One of the odd aspects of the pacing is that the 'action' begins in a state of rest, with the assassin announcing that his trade involves a lot of boredom. He then takes what, in the timescales of The Killer, is a lot of time to make one split second mistake, and after that no more boredom, but also a lot less planning and care.
One last oddity worth noting: it is never explained why the clean up team just gave up. Whatever their expertise they seemed to lack the paranoia which saved their prey from their initial attack in the Dom Rep. Were they worried about a local investigation? The way the movie concludes it is clear that their own nemesis is not worried about the cops looking into the fate of a dead cabbie.