I thought Age of Ultron was absolutely fantastic. It faces a tough comparison to both The Avengers and Guardians of the Galaxy because those two films were unlike anything audiences had seen before, so in that regard it falls short of the very top tier of the MCU (which I reserve for Avengers, Guardians, and Winter Soldier). It's not going to surprise you. You know going in exactly what it's going to be like, and it executes on that promise very well.
Story - Probably the film's weakest point. There were parts where it felt like it was jumping from one battle to the next without much narrative flow in between, but such is the nature of the genre at times.
Humor - Spot on. Appropriate timing. Several laugh-out-loud moments.
Characters - For me, this was the movie's strongest point. Going back in my head, Cap, Stark, Thor, Banner, Widow, and Hawkeye all felt well explored and well developed. It's a real skill to be able to balance six "equal" heroes in one movie without each one feeling shallow, but they pulled it off somehow. Ultron was my favorite MCU villain so far and it's not really close (sorry, Loki). Loads of cameos and minor roles from Fury, Hill, Falcon, and Rhodey helped fill out the universe. The Vision shows up late for narrative purposes but he's freaking badass. He even does the phase punch that he's done in the comics on numerous occasions. The only slightly underdeveloped characters are the Maximoff twins, who are treated as a package deal ("the twins") for most of the film and not as individuals in their own right.
Place in the MCU - A common thing I've seen in negative reviews of AOU is that it spends too much time setting up Phase 3 of the MCU. I disagree. If you like the world building and interconnected nature of the MCU, you'll love those bits. If you're looking to hate it, you'll certainly find something to hate. But if you just don't care one way or the other, I don't think the world-building story lines will either help or hinder your enjoyment of the movie for its own sake.
ETA: I did notice some slight inconsistency in the CGI. There were times when Hulk looked absolutely life-like and other times when even Captain America felt cartoonish. There was also a bit of what I call the "Alice in Wonderland" effect where a felt like I was supposed to be watching in 3D and was missing something for not doing so.
If you haven't heard, there's a mid-credits scene but no post-credits scene, so don't bother sitting through all of them.
