I'm still working my way through this book, and it is painful when I need to stop and take a break. The action is starting to pick up and I need to hide somewhere quiet (preferably a Starbuck's) and knock this book out today.
I did take a break yesterday, however, to play around on the fun interactive site that is set up to mimic Brady's basement on the official Stephen King website (the fun can be found here: http://stephenking.com/promo/mr_mercedes/the-basement/). There are lots of fun puzzles and hidden activities to be found here, but be warned - if you haven't read the book at all, some spoilers will be revealed. If you're a reader that wants to come at this book with complete ignorance of the plot, do not go to this site until you're at least half way through the book.
Also of interest is that the movie rights to Mr. Mercedes have already been sold (back in May before the book was even published) to Temple Hill and Media Rights Capital and director Jack Bender (Lost, Under the Dome, Sopranos) is currently attached to the movie.