I liked it. It did a great job of building suspense, and it managed to show the fright and emotional horror of war without gratuitous gore.
The Spitfire gliding at the end was a little over the top. The real pilot the character was based on landed the plane gear up and walked to the loading area where he got onto a ship, which makes a lot more sense than the movie.
Several ships that were in the movie participated in the real Dunkirk evacuation. That's pretty cool.
http://www.historyvshollywood.com/reelfaces/dunkirk/