Well, I'm probably going to step on some toes, but my thoughts anyway...
**caution** slight spoiler alert
Overall, good movie, I liked the thematic elements and how "real" the screenplay feels and puts you in the middle of the scene. Some of the shots make you feel like you are literally in the lap of the actors or in the scene with the characters. I think Nolan did a good job on that part.
My biggest peeve was that the timeline was hard to follow. You get one quick snapshot of the timeline near the beginning of the movie, but after that you are left to figure out how everything is playing out (from the perspective of the 3 different groups of people, that's all I will say on that part to not give too much more away). You really don't gain a full understanding of the timeline until the climax of the movie. That part frustrated me to a degree from other storyline movies, especially "war story" movies such as saving private ryan. To me it was difficult to piece the story together without a better understanding of the context (if you are going to follow 3 different timelines). You really don't know what the enemy is doing or has done without a little bit of a history lesson before the movie. I dunno, maybe it was Nolan's intent to simply focus on a small piece of the Dunkirk story. If so, then he did a pretty good job of that.
If you have gotten this far, you should still go see it. But it is unlikely a movie that I would purchase. IMO.