In short, this is the novel about the gruesome story behind the disappearance of one member of the wealthy Swedish family and the investigators' ethics, considering their knowledge, how they obtained it and what should they do with it.
This is the novel really worth of your time. Not usual, certainly, there will be some shocking scenes and descriptions. The style is also unusual with a lot of references to modern technology (and brands -- IMHO, that should have been cut out - it isn't really important, it just irritates, as any kind of advertising does). But overall, really interesting read, and definitely not undermining the intelligence of the reader.
I've heard a lot of admiring remarks about main female character, Lisbeth. She indeed has unique personality, but I felt some inconsistencies regarding the way she is described, as I've read and seen some things before, about such issues as she has - probably the story about her develops better later, in the next two novels.
I liked very much the descriptions of Swedish countryside, actually all the descriptions of such nature - I could understand what the author was talking about, I could feel myself being there on the spot. And that's what I regard the real value of the writer and of the novel - to be able to transfer the reader (for a couple of hours or days) into another (fantastic or realistic, doesn't matter actually) realm.