I really, really, enjoyed this. Out of the four Marvel productions thus far I'd say it's definitely neck and neck with the first Iron Man. It's genuinely entertaining and moves along at a fair old pace.
I've got a few quibbles, namely that the biggest and best action sequence happens within the first thirty minutes and nothing ever really matches it. I think Branagh has a better natural eye for action than Favreau and all of the action sequences are a lot of fun, but the Thor, Warriors 3, Loki, Sif vs. an army of Frost Giants battle is just great. The clips released online thus far really undersell the film, particularly the scene of Thor fighting at the hammer site. That's a bad moment in an otherwise great sequence and the pains the online clip went to edit around Hawkeye is ridiculous. The cast is uniformly great, but they're given very little to do. Even Portman as the main love interest only gets about twenty minutes of screentime, meanwhile Kat Dennings is on screen for five minutes...if that.
Chris Hemsworth is fantastic though and in terms of presence and charm he's almost up there with Downey Jr. Just a fantastic presence in the centre of the film and he's the reason the film works as well as it does. He's ably matched by Tom Hiddleston as Loki who is just brilliant as Loki, he's a loathsome, pitiable, genuine threat to Thor and Hiddleston makes every character beat work. He's a character who is deceitful by nature and it plays perfectly against the more physical heroes. In fact I kind of love that his deceitful nature means that we're not even really sure of what he's really trying to do until right at the end.
The set design, of Asgard, is amazing. There's an initial shot which travels through the solar system and up through Asgard which is kind of phenomenal. It's a real thing of beauty, and the film is just full of great looking shots. Branagh really makes the Asgard stuff work, finding an emotional core to scenes that could come across as Flash Gordonish. It's also funny, really funny. Not quite as laugh out loud as Iron Man, but it's got an amiable charm and some great jokes.