After seeing all 18 films in the MCU prior to this, and liking most of them, I was pretty confident this movie would not disappoint.
Other than a few minor issues - that are all my personal taste and in no way reflect on the quality of the film - it really delivers. Thanos is indeed the big, bad villain of the MCU that we've been antcipating.
The Good Stuff
This movie really goes out of its way to explain and even show Thanos's motivation and backstory. I've read some reviews that say his motivation in this film doesn't make a lot of sense, that his motivation in the comics, that this film draws inspiration from, is better, but I disagree.
In the comics Thanos is trying to kill half the population of the universe to impress a girl (a character called 'Death'). Totally boring, somewhat selfish, typical, non-caring, evil villain.
Infinity War has him trying to solve the problem of an unsustainable universe through population control (not really a spoiler). That's a cause, you as an audience member can at least understand, even if you can't get behind his solution. Essentially he thinks he's doing the right thing for unselfish reasons that he believes are necessary.
Way better than killing people to impress a girl. You may even start to care about him a little because of it.
The balance of screen time with all the characters is a master class of storytelling. It's hard to believe people initially had concerns about balancing the storylines of all the characters in the first Avengers movie.
What's even better is the different pairings of characters and how they interact. For example, you get to see more of Peter Parker and Tony Stark but then you throw Doctor Strange, Starlord, Drax, and Mantis into the mix and it's becomes an even more fun dynamic as everybody tries to figure each other out.
The special effects and CGI are outstanding. At no point in this film did I notice any seams between what was real and what was CGI. Thor Ragnarock and Black Panther both had a few dodgy CGI moments but Infinity War is stunning. Particularly with so many CGI characters, this thing is more animated cartoon than live action much of the time but all the CGI characters seem very real.
I enjoyed all the humor in this film. Sometimes it's used to take the edge off really dramatic moments, which some reviewers have said can be a little jarring but I didn't find that to be so.
As most people anticipated the stakes in this movie are real. Several major characters do indeed die throughout the course of the story. It's sad but it's great at the same time... and we know these are comic book movies. No one is ever truly dead.
Minor Issues
With almost every major superhero character from past MCU movies in this film it's inevitable the ones you really want to see probably won't get nearly enough screen time for you.
That isn't a bad thing in terms of the movie. Both the writers (Christopher Markus, Stephen McFeely), and the directors ( Anthony Russo, Joe Russo) have done a stellar job of balancing all the characters so that the time they do have on screen isn't wasted. They either move the story along or provide a great action beat (or both). It's just that, no matter which character is your favorite, you're going to wish they had a bit more screen time (bring on Part 2).
My next, really, really minor issue is a joke with Drax that doesn't really pay off. It's such a fun moment, that I think it was probably a flip of the coin as to whether it should stay or end up as a deleted scene. I'm kind of glad it's in there but there's no pay off later. Maybe the pay off ended up being cut?
I only mention it because the scene stuck in my head as such an odd, awkward, but funny sequence. You'll probably spot it when you see it. It's not really a spoiler since it's a joke that goes nowhere later, and even if it did, it would only be to provide an additional joke.
Last issue, the ending. Don't worry, no spoilers here. Obviously, with a Part 2 coming out in May 2019 this film was destined to not tie everything up in a nice neat bow. Thankfully it's not just one movie split in two. It has a proper end with a set up for part 2 but all the same, I really don't like the year wait to find out what happens next.
To Sum Up
Overall, I enjoyed Avengers: Infinity War. It really does deliver a great Marvel villain, along with a well told story, filled with trademark humor and comic book action brought to life. This movie really is Marvel Studios at the top of their game.
Definitely see it on a big screen if you can. It looks amazing and you'll have a great time seeing your favorite characters doing their thing.