This is Marvel Studios' 22nd movie. And if you're not familiar with the other 21, don't even step into the theatre. Despite a three hour runtime, no concessions are made to the uninitiated, no time is spent on back story. And they reference damn near every character and event in their back catalogue.
I saw the movie in 3D, with D-Box. D-Box at Cineplex is an expensive experience: even on Tuesday, your ticket price goes from the regular $12 to $20. But you get assigned seating, and a very large subwoofer built into your chair. I have to say I did not enjoy the experience: on several occasions it seemed more akin to having someone behind me kicking my chair - really, really hard. Here's the thing: imagine you're in the midst of an on-screen conversation between two characters, and suddenly a third character drops a book. Yes, it's startling, but according to D-Box, your chair just jumped an inch in the air. I get chair-rattling when an airplane or spacecraft is taking off, but they use it to emphasize relatively mundane events that wouldn't in real life cause that level of reaction. The seats also tilt during some scenes. It distracted more than it added to the experience. Even setting aside the price differential, I wouldn't choose to repeat the experience.
The remainder of the Avengers (a term that's now apparently expanded to include not only the Guardians of the Galaxy but also Ant-Man, Spider-Man, and anyone who has winked in their general direction) struggle with the fallout of Thanos' "snap" (see "Avengers: Infinity War" ... or don't see it, which was kind of my recommendation: I really didn't like it much although it's impossible to see this one without it).
I can't tell you a whole lot without spoilers, because by about 20 minutes in I'd have to say "Tony Stark invents time travel," which is A) ludicrous, and B) opens so many logical problems with the entire MCU that I really don't think they should have gone there. But they did, and that means the fight to restore the universe after "the snap" continues, across time. Inevitably the Avengers win, but the victory is bittersweet - unlike the deaths in "Infinity War," I think they plan to make the ones in this movie stick (okay, one dead character has a spin-off movie coming up - but I suspect it will be in the MCU "past").
I'm happy to say that I enjoyed the movie. I have bucketloads of issues with it, but I did kind of enjoy it. If you're a fan of the MCU, you have to see it, it's not really a choice. Do your homework beforehand and re-watch any of the previous 21 movies that you don't remember well.