1917 is very very weird. Not because it is a movie about the horrors of war. I respect that theme, but we have many of those, and 1917 certainly is not the best of them. Depicting both the scale of the carnage of World Wars and the personal level of each individual atrocity are often diametric goals, and 1917 doesn't break new ground at either of those ends. It's good if you just want *more* of that, but not unique.
What makes it unique is, well... it's not a movie.
There is a certain type of videogame that's become increasingly popular, that never provides a gap in the story flow. Cinematics are all done with the game engine. You don't end one level with a boss and portal or helicopter and then start a new one in a different zone with a "Welcome to Green Hills Zone 1" sign, but rather you continuously flow from one room to the next with smooth transitions, so that it all feels like one uninterrupted ride on a lazy river. Black Ops, Brothers, Journey, Inside are all examples. (Yes these sometimes have loading screens, but you are meant to feel that your character did not skip over anything. Open World games could be like this, but it's single player games that can manage the linear on-rails aspect of this story.) They're also slightly surreal, as you see a world change radically and develop in your 120 minute session of sitting in front of the screen with no time skips.
These stories make somewhat more sense for the videogame format, as after all you are inhabiting one person and so just sticking with their point of view is more "realistic." It doesn't feel real to play, just because that's not how we are used to stories by this point, but it does feel hypnotically surreal and is kind of its own genre at this point.
1917 is just one of those games. There's not interactivity, but the story flows much, much more like Black Ops or Inside than any other movie. There's an obvious comparison to classics like Before Sunset which also do the "one continuous take on two characters" type of story, but for those the dialogue and the relationship between the characters is the focus. Whereas for 1917, and the videogames, the *environment* is the focus. We're using this third-person-limited view to explore several epic scene-pieces, and the awe as you slowly come upon them.
1917 does this very, very well. To use the word "beautiful" is an understatement. It's like if the grandest fantasy saga videogame that is drunk on using its new level of graphics to depict every tree and broad horizon, was advanced thirty years. That's what this movie looks like: the storymode of a videogame from thirty years in the future, showing you the beauty and horror of a countryside in the middle of war.
(The plot doesn't help either. Since it wants to talk about a global scale war, but also how one soldier on one mission can make a difference. And you have colorful NPC's who pop up for three minutes to advance your quests, say a couple memorable lines, and then never appear again.)
You should absolutely watch it for the aesthetic experience (on as large a screen as you can manage.) You should also play Journey, for pretty much the same reasons. Those two experiences will be much more similar than comparing it to any other movie.