UPDATE: As of about 7:30PM ET, Thor's approval rating had dropped to 90% based on 70 positive reviews out of 78 reviews total. Regarding the "top critics", the movie has an 88% approval rating, which means that only 1 top critic out of 8 top critics total has given the film a negative review.
As of about 4:00PM ET, Marvel Studios' "Thor" has a 91% approval rating on the popular film review aggregator and forum known as "Rotten Tomatoes". So far, the movie has 68 out of 75 positive, or "fresh", reviews while 7 reviews are negative.
For a movie to be "certified fresh", it must have at least 40 reviews and a 75% fresh rating or higher.
Meanwhile, only 7 top critics have submitted reviews for "Thor", but all 7 have been positive, giving it a 100% fresh rating with the top critics.
Although it currently holds a "certified fresh" rating and has an approval rating in the 90's, these things could both change either for the better or for the worse since many more reviews will be piling in as the week continues. Keep checking rottentomatoes.com for the latest reviews of "Thor"!