It starts off well, with a little glimpse of a British Soldier that was as unprepared as the UK for the war. It's a nice teaser...
...then a little story about the civilian involvement with the evacuation...
...And it all comes with the promise of being a British "Saving Private Ryan."
Unfortunately it ends up being a lot more Michael Bay than Steven Spielberg. that is to say that it looks nice and sleek and professional but is otherwise lacking in a real story.
Spielberg managed to tell a story that was both pro-Soldier and anti-war, and that made the whole Normandy Beach until the very end a pleasure to watch.
Dunkirk, on the other hand, gives up about 15 Minutes in and decides to just focus on the action with the story being an after thought.
In short it's boring.