This movie leaves me with mixed feelings. I can't even describe it as mediocre, because what it does well, it does GREAT. But it's like 1/3 brilliant to 2/3 lazy, and it's FRUSTRATING.
First, the good stuff.
-- Ultron is, unexpectedly, my favorite part of the movie. He's one of the MCU's most memorable villains: he's bizarre, he's funny, he's conscious of supervillain tropes and deliberately avoids them, and he's scary. If anything, I wish he was SCARIER, but in a weird sort of way, his amiability makes him creepier than if they had just gone with a generic "kill all humans" murderbot. When he cuts off Klaue's arm, Ultron actually seems surprised, as though he hadn't meant to maim the guy and is as surprised as Klaue that he just did that.
-- The Clint/Hawkeye scenes are all great, and it's obvious that Whedon enjoys writing the character. The scenes with his wife and kids have a sweetness to them that can't be faked.
-- The Bruce/Natasha stuff is fine for what it is. I disagree with a lot of fandom on this: I can totally see why Natasha would fall for this guy, and I like that she's the aggressor when it comes to pursuing a relationship with him. I feel like this romance was developed as a way to give Natasha an active choice, something the character has never really had. Up till now, her story has given Natasha the choice between 'kill people for the Red Room', 'kill people for SHIELD', or now, 'kill people for the Avengers'. That's not much of a fucking choice to give a strong independent woman who don't need no man. At least in wanting to be with Bruce, Natasha displays some initiative for herself that isn't just who she chooses to kill people for.
-- At the same time, I can understand why Bruce would be so reluctant to put her in harm's way by being with her. I don't see that scene of Natasha revealing her sterility to him as being "oh I'm a monster because I'm sterile" the way some have interpreted it. She's saying to him that not being able to have kids is not a dealbreaker for her, because she's in the same position. Natasha doesn't think of herself as a monster because she's sterile -- she thinks of herself as a monster because killing comes easily to her.
-- The Hulkbuster vs. Hulk fight is well done.
-- Tony and Thor trying to one-up each other by bragging about their girlfriends is very in-character for both of them.
-- Nice touch at Steve being the one to express sympathy for the Maximoff twins and their plight.
-- For that matter, the Maximoff twins telling Ultron the story of their parents' deaths and why they hate Tony Stark is decent enough for an exposition-y bit.
-- The Vision looks cool.
The bad:
-- There is so much set-up and so little pay-off in this movie. Why isn't there a scene where Tony is confronted by the Maximoffs and has to face what his warmongering past did to them? It never happens. Not only that, but there is seriously no reason either twin should want to join the Avengers. I get why they agree to take down Ultron, but neither Pietro nor Wanda have any reason or inclination to join the Avengers, and yet they do. WTF.
-- No scene where Ultron possesses the Iron Man armor. Tony flies around in intelligent metal suits, and Ultron never bothers to take advantage of this? WTF.
-- What is even the point of killing off Pietro/Quicksilver? There's not even a scene of Wanda mourning him. It seems to happen purely so Whedon can have his Whedonism and kill somebody off. I'm inclined to think that Whedon was bitter enough about this being his last MCU project that he killed Pietro out of spite, to keep a future director from getting to use the twins as a unit.
-- Thor seems pretty aimless. You get the impression the script was trying to find something for him to do. Cap doesn't fare much better; he just ambles in to nod approvingly or twirl in the air or give Tony the conflict and validation that he needs, by turns, without any rhyme or reason. It's more obvious than ever that Whedon has NO IDEA how to write either character. This is not surprising, as 99% of fandom can't write Steve or Thor either, but what is disappointing in a fan writer becomes infuriating in a highly paid, trained, experienced director.
-- Tony does a LOT of talking without ever saying very much. I think you could cut 80% of his dialogue without losing a thing. It's like the more Whedon tries to explain his motivations, the murkier Tony's motivations actually become.
-- The momentum from Cap 2 sputters out, as Cap and Falcon play pool and rub elbows with the Avengers. Bucky? Who the hell is Bucky?
-- The Avengers fight gazillions of robot mooks (who are seemingly as easily torn apart as tissue paper)... just like they fought gazillions of alien mooks in the first movie.
-- The audience snickered at the Ant-Man trailer. Not even giggled. Snickered.