It was fun. I wouldn't call it one of the best, but it was a good time. My mom didn't like it, but she was also kind of meh about "CA:Civil War", disliked Guardians 2 and liked Thor 2 (she and my sister actually made squeally noises when he appeared on the balcony to make out with Jane). She did say she would like to watch it with subtitles, because of difficulty following the dialogue, and doesn't like fight scenes as much. She is also the only person in our family who regularly ever read comic books enough to know something about these people. So take all that for whatever you think it's worth. It's directed by the Russo brothers, with their excellent action sequences from the last two Captain America movies. As Avengers directors go, YMMV but even if you don't think they ever matched Avengers 1, they've been more consistent than Whedon, and all their films have been much better than Age of Ultron.

So the basic plots of the movie are (loosely, not spoilers, except for who's in the movie and what are they doing, very broadly speaking)
A: Thanos, with a bunch of creepy guys who look just enough like they belong in the MCU that you're sure you've seen them in other movies (but you haven't), is going around killing people to get his hands on the Infinity Stones in order to enact his objective, because the Infinity Stones, when working together in a device made for that purpose (a yellow gauntlet) are powerful enough to let you do anything. And he actually has a direct and specific objective, that he's been working on for a long time, but the Infinity method is basically the only way it's going to get done.

What this means, is Thanos has an arc and characterization. He's not just doing this to get power or wealth in some nefarious, damn the cost to others way. He has a sort of hero's journey thing going on, even if the heroes from the other movies are the protagonists and central characters of the film. Josh Brolin does a really good job playing him, too. It doesn't make him any more sympathetic or less horrible than you can infer from what Gamora & Nebula said about him in Guardians, but it gives the movie a degree of plausibility, in my mind, at least. As a character, he's the best Marvel villain since Loki, and he's a lot more of a threat than Loki and not just because of the scale of his evil or degree of power.

B: Thor learns about Thanos' mission, and after being forcibly separated from the Asgardians again (look, if you don't want to tell a story about Thor doing things appropriate to a king, DON'T MAKE HIM A KING, especially when he isn't in the original material, okay? Just have him be a guy from Asgard who fights things that threaten Asgard or Earth, instead of having him keep falling off Bifrost or whatever), encounters the Guardians of the Galaxy. He then takes Rocket & Groot to go looking for a weapon with which to fight Thanos. Rocket tries to help Thor deal with his issues, and there is an encounter that is almost hilarious in its casting. Thor is pretty damn impressive in this one, BTW. Probably his best showing after Thor 3.

C: The reduced Guardians of the Galaxy go off to stop Thanos from accessing various stones, looking for the Collector and then Thanos himself, because its' Gamora's turn for daddy issues. The Guardians weren't as fun as in their own movies, I felt, because Gamora is going through stuff, and Starlord is affected by it, and so Drax & Mantis, while still funny, feel almost out of place. But their well-established shit does give their story a bit more heft than some of the others.

D: Word gets to Earth, where two of the Infinity Stones are in Dr. Strange's necklace and Jarvis/Vision's forehead. Iron Man and Spider Man show up to help protect the former, and go on a pretty neat adventure to protect the stone from Thanos' operatives and Thanos himself, with the similarities of both Strange & Stark clashing and at the same time, resonating with Thanos. And Spider-Man is there to keep the dickishness entertaining.

E: The Avengers have to come in out of the cold for this, and team up with the remnants of Team Stark to protect Jarvis' stone, and that means going to their new home-away-from home in Wakanda, to fight off Thanos' armies and prevent them from getting to Vision & his stone.

Several of these plotlines converge, rather expectedly, since everyone is about protecting stones from Thanos, or going after Thanos, who, in turn, is after the stones.

Who's in it?
Mainly: Iron Man, Dr. Strange, Thor, Rocket, Gamora, Vision, Scarlet Witch
Secondarily: Starlord, Drax, Mantis, Spider-Man, Bruce Banner, Groot
Also appearing: Black Widow, Captain America, Falcon, War Machine, Loki Heimdal, Okoye, Shuri, Nebula, Pepper Potts, Wong, Wakandan Gorilla Dude, Peter Parker's fat friend, William Hurt From Norton-Hulk & Civil War
Name-Dropped: Hawkeye & Ant-Man
Surprise Re-Appearances: 3 (but one is a different actor, so you might overlook it, as I did the first time)
Deaths: That should not matter to how good a story is, but at least three or four that could be pretty real, and not events you can assume are undone based on a look at the next wave of Marvel releases.
Post Credit scenes: One, all the way at the end. It's more of a future movie hook than a funny skit.