Flat out, hands down AWESOME. And I am not a big mystery/thriller reader either. I read this because I heard really good things from a lot of people and Larsson's "Millennium" trilogy seems to be the hot thing in the book world at the moment, but I did not expect to like this book as much as I did.
One thing I can say about it is: Girl w/Dragon Tattoo is way better than the sum of its parts. As in, there are bits and pieces that (in any other book) I would have been critical of. Characterizations that (handled any other way) would have been cliche. Plus, the whole who-did-it? plot is not my favorite to read. But, taken all together, it works in Gw/DT. It kept me interested and (at the end) enthralled with page after page after page.
And you really can't ask for more than that from a book.