I watched Dunkirk twice. It's very good, but it's the kind of movie that I think really requires a second viewing to nail down exactly how you feel about it.
It's not a conventional war movie, which is both a good thing and a bad thing. If you go into it expecting Saving Private Ryan, you'll be disappointed. But if you go into it expecting to see a talented filmmaker, crew, and cast cleverly subvert traditional storytelling techniques in order to tell a complicated story that has dozens of characters with many events taking place simultaneously, I think you'll enjoy it.
On my first viewing it took me a while to figure out what they were doing and get into it. I liked it more on the second viewing since I understood the structure of the story better and was able to focus more on the details.
Either way, Dunkirk is a movie that will definitely benefit from a big screen and big sound.