Long! So, so long. We took my 9yo son and 6 of his friends to see the movie, for his birthday. My son, big-time superhero nerd, was rapt. But after 20 mins of previews and ~90 mins of movie, some of the other 8/9yos with us got antsy. And there was still a whole nother hour to go! 3 or 4 of the kids made a few trips to the bathroom, or water fountain, or whatever.
Dark! So, so dark. Scenes of torture, a few deaths. Much darker than many/most (all?) of the Marvel movies have been. Grim and desperate.
Busy. You need a pretty solid sense of who all the heroes are from previous movies, because there is absolutely no extra time to fill you in on anyone's backstory.
Choppy. The ensemble is so huge, they have to cut away to what a different group is doing in another part of the story, like every 2 seconds.
I thought it was great. Epic. Ambitious, well pulled-off. I think I may have loved it? But I have to stop short of "recommending" it, esp to families with kids younger than ~10 or so, due to reasons given above. Not as fun as Thor Ragnarok, or Doc Strange, or even Wonder Woman. That's right, less "fun" than World War One. But yeah, I thought it was great. I was impressed.
“War is the remedy our enemies have chosen. Other simple remedies were within their choice. You know it and they know it, but they wanted war, and I say let us give them all they want.”
― William Tecumseh Sherman