I saw it in IMAX and thoroughly recommend it. Historical inaccuracy aside it was an excellent movie and I thought it did a good job of capturing the overall spirit of the event without trying to encompass the entire thing which would have become a 4 hour "The Longest Day" in reverse. You follow a couple groups of people through their intertwining experiences in a snippet of time in a snippet of geography in what was an extremely large event. It's not dialogue driven though you see some evolution of characters over the course of the film. I really only found 2 "are you serious with this?" moments in the film, one was relatively excusable the other was blatant. I won't spoil. You should be aware that the film does not flow from point A to point B in time, it jumps back and forth and you will see the same events from different perspectives several times so don't get all bent out of shape if there seems to be continuity problems, There aren't. It's supposed to be that way. Also, I feel the IMAX really added something to the film. I know it was shot on 70mm film unlike most modern movies so I don't know if that was part of it or not but it seemed like it belonged on the huge screen as opposed to some films where it's added on and it becomes difficult to see everything.
It was great and I think the moving through time added to the sense of dread, despair, desperation, and foreboding when you see the characters in a struggle which you already know won't end well for them.
Didn't find it to be depressing at all.