"Thor" had it's official premiere yesterday (today?) in Australia, where star Chris Hemsworth got his start as a popular TV actor, in preparation for it's UK rollout on 4/27 and it's U.S./etc debut on May 6th. It was also, apparently, shown to some U.S. critics around the same time. The reviews have started hitting the web and Twitter... and so far the word is good. Hollywood Reporter liked it, so did Variety. On the "geek press" side, Drew McWeeny agrees. There are a bunch more, but these will give you a rough consensus.
UPDATE! Empire's review is another rave.
Those trying to stay 100% spoiler/plot-structure-info free will probably want to just skim those or avoid full-reads altogether (nothing major in them, but you never know) but a few consistent themes run throughout almost every review I've read:
It's bigger than we've been sold. The trailers have been short-selling Asgard and focusing on the Earth scenes. Apparently this doesn't reflect the "ratio" of cosmic-to-earthbound action in the actual film.
It's not a "dark" movie. This intrigues me - almost every all of the reviews that bother mentioning the other Marvel/Avengers films make a point to mention that this is probably the most "kid-friendly" of the cycle so far; in as much as it's more heavily grounded in comic/fantasy business than PG-13 body-horror ("Incredible Hulk") or midlife-crisis and/or corporate politics ("Iron Man 1&2.") I'm MORE than fine with that - I don't want to see a "grownup" movie about a Space Viking looking and his Magical Hammer.
Hemsworth is good. This is the first Marvel/Avengers movie that's being shouldered by a previously-unknown leading man, so it's good to see mostly across-the-board praise for Chris Hemsworth.
The drama works. Another VERY frequent compliment - though often noted as the "upside" to criticisms that the action/fight sequences aren't necessarily the most groundbreaking. Probably the single most-recurring thing in the first crop of reviews is near-univesal agreement that the courtly/family intrigue among The Gods kills; with ample credit being given to the choice of Shakespearean/arcane-bombast specialist Brannagh as director. This is GOOD to hear, if it's true - "Thor" is both the weirdest and least-known of the principal Avengers, and if this film AND its/his part in future continuity is going to work people need to grasp/"buy" the characters and their world. Remember: "Iron Man" worked so well as a superhero character-study that no one cared about (or now remembers) it's awkward, nonexistant 3rd act.