I just read the Millennium Trilogy a series of three thrillers by the Swedish journalist Stieg Larsson. Obviously the books have been around for a while but only recently became available (without reserving them) at local libraries.
In my view the books aren't all that good. They are quite long (the first is 644 pages in paperback, the second and third 503 and 563 pages in hardcover) and seemed uneven to me dragging in places. The second book contained a number of implausible coincidences and a preposterous (even by the standards of the genre) escape from peril. The Lisbeth Salander character is quite unrealistic. Annoyingly she is portrayed (briefly, again in the second book) as a brilliant chess player by an author who clearly knows nothing about chess. The books are set in Sweden and contain a number of obscure local references. This is partly the fault of the translation, for example it would have been simple to give the approximate value of the Swedish kronor in dollars when it was first mentioned.
I wouldn't say the books are terrible. I did find them interesting in parts and I have certainly read worse. However I have also read many thrillers as good or better that have not achieved the same popularity. The ingredients of popular success are a bit mysterious. In this case I suspect Larsson's untimely death contributed as did the somewhat exotic setting (Sweden). Perhaps in some way the books resonate with a portion of the reading public. And of course once a certain level of success is achieved a snowball effect can take over.
In summary these are the sorts of books you might read on say an airplane flight if nothing better was available but not really anything I would recommend making a special effort to read.