
The bottom line: I'd put it a safe third in the Marvel Cinematic Universe films (behind Guardians of the Galaxy and the first Avengers, but definitely ahead of Captain America: Winter Soldier and Iron Man 3). There's a lot in this film, you'll want to be paying attention, and you'll definitely want to see this movie with a nice big crowd cheering on your protagonists.
Joss Whedon isn't at the top of his game here, but he is pretty close, and it's a good time.
Don't worry, the spoilery parts are all safely below the jump.
We start with the Avengers doing what they do best: working together to tie up some loose ends from Captain America's (Chris Evans) last outing with HYDRA. Everyone's back, of course: Tony Stark (Robert Downey, Jr.), Thor (Chris Hemsworth), Dr. Banner (Mark Ruffalo) and his big green alter-ego, Hawkeye (Jeremy Renner) and Black Widow (Scarlett Johansson), and they bring the fight beautifully in the opening sequences.
What they discover is a certain Asgardian artifact that played a large role in the first Avengers film, and how it was used to create some impressive technology, including human experimentation on twins Pietro and Wanda Maximoff (Aaron Taylor-Johnson and Elizabeth Olson). With the artifact safely in hand, the team kicks back and enjoys some time off while Stark and Banner go into Full Science Bros Mode in order to try to create the ultimate peacekeeping force against the kind of cosmic-level bad guys that have come knocking on Earth's door lately, a project they dub "Ultron" (the outstanding James Spader, who is as perfect a casting choice as Downey is for Stark).
Only with the technology they now have, Project Ultron turns out to be too successful. And boy, does he have daddy issues. What follows is a whirlwind, globe-hopping adventure that will test the Avengers friendships, teamwork, and strength to the maximum, and it's a great time.
Again, I highly recommend the film, as Marvel has done it again.
Now, about those spoiler below the jump...and you have been warned...