It was great.
It seems that fans liked it and critics were lukewarm about it. I came out of it thinking the fans were right and the critics not so much. The critics compared it to Guardians of the Galaxy, Winter Soldier, and the first Avengers movie. My initial reaction was that it was just as good as those movies.
Then I started thinking about it more. It’s a good movie, but there is something that makes it perhaps lesser than the other three. Age of Ultron feels like a great middle-third of a book. The other three movies you could come to without being invested in the characters or world, and get sucked in. This movie, I feel, you need to be invested in the characters and storyline for it to work. If you were coming this without exposure to the other content, it would probably seem like an action-filled mess.
As it was, I enjoyed it very much, particularly the rendition of Ultron.