Dunkirk is, as of this writing, getting 92% positive reviews on Rotten Tomatoes. I'm with the other eight percent.
The movie has many positive qualities, by far the best of which is the cinematography. I was lucky enough to see it projected in 80mm, which definitely gives a better experience than standard 35mm or digital projection. The actors all delivered fine performances. The sound was masterfully mixed.
Unfortunately, none of that makes up for the film's two enormous flaws:
First, the narrative is badly disjointed, jumping back and forth across the timeline as it moves from story to story. Some of this is caused by the plot including so many disparate parts, but that's no excuse. Christopher Nolan, who wrote, directed, and produced, has moved well past artistic license with story structure (which he did well in Inception) into needless muddling. It is very possible that an editor could create a version of this move which works much better than the one I saw.
Second, the vast majority of the characters lack... well, character. This isn't the fault of the actors - as I said, they did a fine job. The problem is the script, which completely fails to give the audience reasons to care about the people on screen other than the direness of their circumstances. Most of them barely have names, and none of them have any evidence of interesting qualities other than the desire to survive - with the welcome exception of the featured civilians.
So what we end up with is an engaging, immersive film that utterly fails to be compelling on a personal level (except for the small percentage of the movie which focuses on said civilians).
Slight spoilers in the last paragraph:
Aside from the previous complaints, I have one other, lesser one: the enemy is faceless. They're hardly referred to as Germans at all - the opening text calls them "enemy", and only a couple lines of dialogue name them. While the effects of the German siege are depicted frequently, at no point is any German face depicted in focus - only airplanes, bombs, and bullets. Perhaps this was done to avoid the appearance of offending modern Germans, or perhaps it was an artistic decision to focus on the British soldiers and their plight. Either way, the lack of a visible antagonist further dehumanizes a movie which already lacks in that regard.