Several excerpts have been published*, mostly since Wallace's death in September 2008 (links here), and now Publishers Weekly has posted a review. I have read all of the excerpts, but I'm trying to avoid learning any of the actual plot details, so I skimmed some of this.
* - "The Soul Is Not A Smithy", a story from Wallace's 2004 collection, Oblivion, may also be part of TPK. (An excellent essay about that story is here.) Looking through the list of stories -- "Mister Squishy", "Another Pioneer", "Good Old Neon" -- I really need to read some of these again.
[T]his isn't the era-defining monumental work we've all been waiting for since Infinite Jest altered the landscape of American fiction. (To be fair, how many of those sorts of books can one person be expected to write?) It is, however, one hell of a document ... a transfixing and hyper-literate descent into relentless, inescapable despair and soul-negating boredom.* - There is a character in the book named David Wallace.
The story ostensibly follows several recruits as they arrive at an IRS processing center in Peoria, Ill., in May 1985. ...
Stretches of this are nothing short of sublime - the first two chapters are a real put-the-reader-on-notice charging bull blitz, and the David Foster Wallace sections* ... are tiny masterpieces ... Then there are the one-offs — a deadening 50-page excursion to a wiggler happy hour, a former stoner's lengthy and tedious recollection of his stony past — but this is a novel of boredom we're talking about, and, so, yes, some of it is quite boring. ...
Of course, this is an unfinished novel. It's sloppy at times, inconsistent in others, baggy here, too-lean there, and you rarely feel that the narrative is taking you somewhere. Instead, it's like you're circling something vague, essential, and frustratingly elusive.
In March 2009, Michael Pietsch, Wallace's editor, told Entertainment Weekly that Little, Brown would be posting large sections of the manuscript (as well as earlier drafts and notes) online, so readers can get a "detailed sense of Wallace as a working writer" and see how the book was compiled and edited. There has been no subsequent news about this website, so I hope LB follows through on its plan.