Avengers: Age of Ultron is not as good as the first Avengers. That is, however, a pretty high bar, and I expect Ultron will finish the year at least near the top 5.
I've actually read the Age of Ultron arc in its original form. I expect there are some trufans who are not on board with what Whedon and Marvel have done with it, but I didn't mind. Sure, in the books Ultron was created by Hank Pym, but a) he's not a character in the Marvel Cinematic Universe yet, and b) Tony Stark is positioned in a fairly similar niche. There are numerous other changes as well, but they all make sense to me.
Ultron is, of course, an action movie, and it has some very nice action sequences. The effects are superb, the scope is suitably large, and everything feels solid and consequential. No new ground is broken, but the old ground is well tilled and producing lushly.
As is standard for Marvel movies, much attention is also paid to the characters. This can be hard to do in ensemble films, but Whedon is good at it, and he is on top of his game in Ultron. Each character has motivation, personality, a subtleness of reality that is astounding in a comic-book action movie. It helps that the actors know the parts so well, I'm sure.
As for the new characters, Ultron himself is well-realized both in effects and voice acting, and Vision even more so. Quicksilver and the Scarlet Witch are serviceable, though not spectacular. To the extent that the move has flaws, they are with the main plot, which has some holes. On the other hand, they are gloriously comic-booky sorts of holes, and it is a comic book movie.
Much has been made of the treatment of a certain character in this movie. I like the way she was portrayed, but don't feel the need to add more argument to the discussion.
Bechdel: Pass (A)
Mako Mori: Pass
What are these?