It's a good pop corn flick. It doesn't reinvent the wheel, but it delivers on character development and big set pieces. The main cast shines, but there isn't enough time for the new additions to truly have an impact. The movie's more fast-paced and quippier than the first, and the plenty of fan-service for the masses. On the other hand the pace is too frenetic, and the movie's ultimately not long enough for the story to deliver on all the premises.
As a villain, Ultron starts off on a high note. He's suave, menacing, mysterious ans determined. But in the later half he morphs into a bratty robot-boy, and completely deflates during the climax. And speaking of the climax... well, let's just say I expected more. Much more. It's just too easy for the Avengers. And it's really too bad, because Whedon goes out of his way to demonstrate how dangerous the final confrontation could be, and how much the Avengers stand to lose both individually and collectively.
There's a big takeaway from this movie though : Vision. By far my favorite character, and extremely well realized. Well, save for those eyes. A little too human if you ask me :)
I advise all those who haven't seen it to manage their expectations. Don't go in expecting to be blown away. Hope for some blockbuster fun and a decent story.