There is one statement most fans reading this review will want to read: Avengers: Infinity War is a five star film. For the most part, anyway. It is the best experience of any MCU film - even when it isn't the best put-together story (Civil War, Guardians and The Winter Soldier are more nuanced and well-rounded) it is a spectacle like no other in this franchise or any other.
It ebbs and flows at times and the interest curve is definitely not always at the top of the graph for its entire (very long) run-time, but taken as a whole picture, it is a breathtaking, impactful experience that deserves to be watched a lot more than once. What the Russos have achieved in balance, power and spectacle is nothing short of profoundly brilliant.
After ten years of building characters, seeding one over-arching narrative and bridging an almighty 18 movies together into one cohesive narrative, the MCU has arrived at one hell of a climax. Or at least, one part of one hell of a climax. The fact that we get to see more of this in just a year feels like a treat that only loses some lustre for the fact that we have to wait that long. Then again, given what it takes from the audience by the end, maybe a little period of respite is sensible.
Even with that top rating, the film is not flawless - somewhat inevitably - and it's only because of how great it is that the areas for possible development stick out. And ignoring them would be disingenuous, so that's the best place to start with this review. There are no spoilers within, of course, because Thanos demanded as much.