Just saw it today.
It was OK. Not as good as the first one, but better than Iron Man 2 and 3, both of which I detested.
I think the main problem with the movie was Malekith. He just came off very flat, as the villain. I didn't find him scary or impressive. I didn't hate him. He didn't have one memorable line in the movie. Compared to Loki, who was an outstanding villain in both the first Thor and Avengers, Malekith was entirely lackluster. His "plan" was boring and uninspired.
I also thought the part where Selvig's home-made instruments were able to create space-warps was utterly preposterous. We've spent two whole entire movies (Cap and Avengers) with every super-powered being in the universe including Thanos, Loki, and Thor, along with Cap, the Red Skull, and SHIELD, is willing to wage war to possess the Tesseract, and Selvig creates something that is functionally equivalent in his basement while wearing his underwear? I mean come on....
Loki, I will say, was a joy to watch as usual. I love him... smart, duplicitous, cunning, and you're never sure where he really stands. And Darcy was fun.... she should have had a little more screen time. And Chris did a good job as Thor, as usual.
But on the whole, by my lights -- too many plot holes and a flat villain. It was OK... but it's not one I'm going to bother buying, because I probably won't ever want to watch it again.