Saw it on opening day (matinee showing) and rather liked it. With the usual asterix postscript that I didn't like it as much as the previous one. Which is the film's largest sin: it just wasn't as good as a freakin' amazing bar setting film.
There's a lot of complaints aimed at the movie and I think most of them can be distilled to that. People walked out thinking "that wasn't as good, and so I'm disappointed: how do I quantify this feeling?" Just like The Dark Knight Rises, which had just as many plot holes and weird days as The Dark Knight, but wasn't as good so the problems become something people latch onto to complain. People need to justify and explain away this dislike, and seem unable to rate a movie between "amazeballs" and "terrible".
There was no way Age of Ultron could possibly be as good. Even if the story, acting, jokes, and action scenes were all better it would lack the sheer thrill of seeing all these characters come together at the same time in the same movie.
(It doesn't help that the best action scenes and most iconic shot was at the beginning so people didn't walk out of the theatre with that in mind. Instead, it ended with Captain America *almost* saying "Avengers Assemble" but not, leaving us feeling teased.)
I've seen people complain that Age of Ultron is unsatisfying as a big dumb popcorn movie for not being big or dumb enough.
I've seen people complain about the lack of character growth, as if it were possible to have all the characters change from a single story (and the first movie didn't do this either). This is extra odd since few popcorn movies have any character growth at all, or a circular growth where they have a moment of doubt and questioning after a failure and they end up back where they started.
There's the whole feminist angle, which is sprawling out into people critiquing Joss Whedon's previous work with the relish of attacking a fallen icon. ("Finally I can attack Buffy the Vampire Slayer for being imperfect!") Which seems especially odd when you compare Whedon to most of the other big name directors. Imagine what Michael Bay would have done with Black Widow. *shudder*. Or even Spielberg or Cameron.
The movie does have its problems, much of which can be attributed to the rigid formula the movies have fallen into. The first movie was a culmination. It was a big climax and the future movies reacted to its events. This one is right in the middle of a huge arc that has been building for some time and will continue to build, and does more setting-up of stories than resolving. It's the middle child. How well the movie fares depends so much on how these future arcs and threads are resolved.