Unfortunately not as good as Män som Hatar Kvinnor
. He has gone too far, and Lisbeth Salander is no longer a fully credible person; also, the puzzle isn't as satisfying as in the first one.
Start geek-rant: as a former mathematician, I was annoyed by his sloppiness concerning Fermat's Last Theorem. To start off with, he misquotes it several times. And the whole idea that Lisbeth is able to solve it on her own in just a few months, with no formal mathematical training, is cheap. If this were the only thing wrong, it wouldn't of course matter very much. But it's more a symptom of the lack of care he is displaying... the whole book has an unfinished feel to it.
But, before I get too critical, I must admit that I couldn't put it down, and that the main characters, especially Lisbeth, are wonderful creations. I'm sure I'll read the third volume soon.