I am very happy to say that it was fantastic. It followed up nicely from the Avengers, being sure to continue from the first Thor too, making a very personal story about the Asgardians and Thor and Loki in particular.
Beware, minor SPOILERS ahead.
Events start off with a piece of history, and then introduces us to various worlds of the nine realms. We then follow Jane Foster as she tracks down some strange readings in London (played by the always wonderful Natalie Portman as before). There's a lot of humour and fun in this part of the movie, especially with the character of Erik Selvig. Shortly though, things begin to go wrong and Thor travels in to save the day, taking Jane Foster back to Asgard with him.
Once there, events slowly taken a turn, with the resurgence of a great evil and a spectacular action sequence as Asgard is attacked. Events conspire that force Thor and Loki to work together, fighting against Malekith. This is followed by the awesome set piece finale set in Greenwich in London and then some interesting revelations before the closing credits.
Oh, and just so you know, this movie has two post credit sequences, one after the stylised credits that nicely sets up Guardians of the Galaxy, and a second scene at the very end of the credits.
Overall, an absolutely great movie. It wasn't as exciting or action packed as the Avengers, but then it was never supposed to be. If a threat is so great that it needs all of the current roster of heroes to deal with it, it wouldn't work in a standalone movie.
I give it 9/10
Now I have to wait for Captain America - The Winter Soldier!