I loved it. I don't follow the comics, never read them to be honest (though I would like to start), so I don't have any basis for whining about plot changes or whatnot. I'm going in blind and I loved every second of it. The effects, acting, the characters, Whedon's amazing script and the way he writes such natural interactions and hilarious jokes - this is what I love about the Marvel movies and especially The Avengers. The action I could take or leave, but with SFX the way they are now it's always a joyride to watch these movies. We've finally come to the point I think where what's on screen is as good or better than what I can imagine in my own mind for any given (written) scene.
I didn't have any strong gripes - Ultron is a better villain than most, it's just too bad nearly all villain's motivations are A. revenge or B. wanting to take over/destroy the world "for everyone else's benefit". Actually it was kind of both this time. So predictable but I wasn't going in expecting to be wowed by that aspect of the story. I am glad I ended up liking Quicksilver and Scarlet Witch so much; with the hype their addition to the movies caused I was afraid I wouldn't think much of them, but they turned out to be sympathetic characters with interesting powers. They added some new moves to the group that were really fun to watch.
I do wish that a little more came from the "dreams" SW gave the Avengers. Especially with how Tony's dream played out, I really wanted him to have a moment near the end of the movie, but I guess the movie isn't solely about him and he kind of had his big identity moment in IM3 anyhow. I do feel like a bit more could have been done with that anyhow.
Definitely looking forward to the end of the story of the gems and how that all goes down. Cannot wait.
Btw, did anyone else notice billing for the Ghibli Laputa robot in the credits? I didn't even see it in the movie...I wonder where...