I had trouble staying with this one, and I can understand why the other reviews here are rather polarizing. I went into it not knowing much of anything other than that P.T. Anderson has adapted it into a film, and I wanted to read it before I saw the movie. I have also never read Pynchon before, and this may have been an odd book to start with as it is apparently outside of his usual style.
The plot was tricky to follow in the sense that it begins as a fairly straight-forward mystery, but things get complicated when character upon character are introduced and it's not immediately apparent if they are important to the story or merely colorful distractions. While things eventually tie together and you come to learn who is really connected to the plot and what details are important, there are also plenty of stories from various characters that are ultimately pointless other than that they represent the culture and attitude of the 1970's and are often funny in their absurdity. The book is funny, without a doubt, and has enough pop culture references to keep things interesting most of the time, but following the mystery can get muddled when Doc Sportello is frequently stopping to get high and there are long passages of drug-addled existential gibberish.
I can see the pure talent of the author in this book, and I do believe that technically it is a good piece of work, it just felt a little high-brow for me (somehow absurd humor and satire can also be too literary for its own good). While it started as a fun and very funny book, it ultimately became tedious and tangled. High-profile critics who raved about the book would probably just say I was too daft to understand it, and perhaps they're right.
That said, I'm fascinated to see the film for a couple of reasons: 1) I think it would be extremely difficult to adapt because of its meandering narrative, but apparently P.T. Anderson's script has Pynchon's personal approval, and 2) I have yet to see P.T. Anderson go awry. I believe he is an incredible talent who can tackle every genre he takes on, and is on his way to be the Kubrick of this generation. Also, the cast is exceptional. Really looking forward to this one in December.