It is going to be quite difficult to review this without spoilers, so this post is also the debut of the new spoiler button. Click it to see all the spoilers.
This film is what the MCU has been building up to over the period of many years, acting as a massive crossover of all of the Marvel streams they've spent quite a while putting together.
That's both a strength and a weakness. Anyone who hasn't seen enough of the previous is going to be massively lost, because the action starts in medias res, with Thanos running into Thor and the Hulk on the Asgardian ship, using the power gem to grab the space gem from the Tesseract. Thanos is now collecting all six of the infinity gems to give him control of the entire universe.
And so begins the first of this two-parter, which sets up Thanos as the greatest foe the Avengers will ever face. The film consists of three main streams, each correlating to capture or defence of a given Gem.
The Guardians of the Galaxy attempt to hunt down the reality gem, while aiding Thor in the creation of a weapon to stop Thanos. Cap and Black Widow work with T'Challa to keep Thanos' agents from the mind gem in Vision, and Stark and Petey attempt to help Strange defend the time gem. Obviously pretty much every other person who's appeared in an MCU drops in, so I'm not going to name check them all for you.
The episodic nature of the film allows a decent range of tone, as the Guardians are funny, the earth is dramatic and there's some nice space horror, both in the fights and some of the sacrifices characters have to make in order to stay ahead of the game. And the shifts are done better than I'd expected, with a pretty consistent overall feel to the film. It does it a lot better than Thor: Ragnarok managed.
In general it's a very satisfying film, with everybody in it having a cool moment, and none of them feeling underused. That does lead to a slight flatness, in that the film doesn't have much of an arc. We know Thanos is going to go around collecting all the stones, and the fact it's a two-parter means you can see how the end is going to go to set up the next film.
Still, getting there is fun, even if it didn't have much of an emotional impact on me. Might partly be due to it being very much in the form of a comicbook universe. You always know things will keep tinkering onwards, eventually reverting to the status quo. That's one of the major objections to Marvel's repeated "events" in their comics, and I think the same criticism applies here. There's a particular example of that in the film which I'll mention in the spoilers.
On the whole, it's not a bad film. All of the characters had something to add, the fights are pleasantly flashy and reasonably varied (but occasionally run on a little too long) and the story is a pleasing twist from some of the previous, with Thanos' motivation being one we've not seen in a while. Looking forward to the second half, but definitely counts as a popcorn thriller rather than an emotional heavyweight.