I recently read the book, which I wasn’t interested in at all but someone gave me a copy so I felt I had to. I didn’t think much of it. I haven’t seen the movie so I don’t know how it compares, but the book has (at least) three major plot threads that have very little to do with each other, and a hero who seems like a pretty average guy but for some reason every woman he meets immediately wants to have sex with him. It’s my impression that Salander is the main character of the movie, but she’s more a supporting role in the book.
As a mystery I felt it was pretty weak – I guessed a good bit of the solution very early on, but IIRC it’s over 300 pages into the book before the hero discovers the first real clue.
The English edition also struck me as being not very well translated. I’m just guessing on that since I don’t read Swedish, but while reading the English edition I kept encountering phrases that seemed “off”. For instance, the word “anon” was used casually at least two or three times. “They had their first quarrel, then others, and anon the antagonism turned personal.” This might be okay in a historic novel or a fantasy that was going for an archaic style, but it seemed really awkward and out of place in a contemporary thriller.