The books are said to (become) a big hit. They're huge. LoTR style.
I must admit I read some reviews, all of them positive, before watching the film, but those didn't spoil it.
Haven't read the books, some of my family have and they considered them ok, but one quote I remember well: the reviewer actually prefers the shortcut the film needs to take to the extensive description of literally every character offered by the book(s).
I wouldn't say I am as lyrical about it as some of the people I've met, but it is definitely a good film. Gripping at moments, a high pace. The style one could describe as typically Scandinavian; cold, winter, bleak colours, but this is also due to the setting of the story.
There is one plot instrumentum that I wasn't too happy with. It becomes obvious that while "sidekick" Lisbeth Salander (she's much more than that, a stronger personality than Mikael Blomkvist, which is part of the whole male vs female discussion that the book/film raise) discovers who the murderer is, he of course is just at the house of the killer. And is caught by him. And yes (no! you groan) she manages to save him at the very last moment.
Yes, go on, give me crap about timing and suspence, but it ruined the fairly realistic tone that was established. This single plot device suddenly turns you around and you see the Hollywood cardboards, the effects, ...
But that sounds too harsh. I truly liked the film.