I was always predisposed to liking this movie: loved the comics as a kid, loved the Marvel movies and enjoy Joss Whedon's stuff too. So what did I think?
First off stupid title: no, not Avengers Assemble it's officially
(in the title and everything) Marvel Avengers Assemble, stupidest title
Iron Man's relationship with Pepper has evolved and instantly adds zing, Mark Ruffalo's Bruce Banner is immensely likable (watch for a really soulful look in his eye as he transforms into the Hulk the first time) and the interaction between all the main heroes is great to watch. Of particular note is Johanssen's Black Widow: slightly cold but actually really convincing as Natasha, a far cry from her role in Iron man 2. Jeremy Renner does well with the little he has to do as Hawkeye and all the heroes bicker and argue (real Marvel style!) through the slightly claustrophobic and enclosed mid section.
Then the film opens out dramatically for the prolonged final scenes, featuring without doubt the best superhero action you have ever seen. One battle scene featuring the entire cast in combat throughout the city is obviously cobbled together digitally but stunning in its breadth of scope. The Hulk here is a real joy: while not quite as plausible as the Ang Lee version, its still by far the best movie version: muscled like the comic version but without far greater skin than the Ed Norton version, any scene with the Hulk is a pleasure to watch...and frequently hilarious.
We expect humour from Whedon but it really zings and adds so much to this movie. This movie will be huge as it was a full cinema on a Thursday, all laughing frequently at the writing.
Spoilers prevent me from revealing much but watch for the Hulk vs Loki scene, Iron Man's first words to Thor and the mid-end credits scene left me thinking "What the hell are they building to?" before letting out an audible "Ooh..."
Takes a while to get going but somebody once said your remember a movie by it's last scenes and this left me very excited and wanting to watch it again...