Telling you that this is a whale of a book containing numerous subplots as well as plots to gain power over the whole planet; a book where laws of physics and mathematics are tested; a book full of funny songs, weird sexual encounters, drug use, and goofy names might sound somewhat familiar if you have read Pynchon before.
But Pynchon has topped himself - even if it is just the length of the book, which is, however, quite an accomplishment ;-). But rest assured, as long as your journey with Pynchon might be, it never gets boring as you take an extremely funny trip through the diverse genres of the historical novel, the western, the epic, the picaresque novel etc. which brings the whole world together - quite literally - and ends on a seemingly sentimental note, which, on a closer look, turns out to be the end with a bang, not a whimper.
While Pynchon's last two novels ('Mason & Dixon' and 'Vineland') were somewhat letdowns, 'Against the Day' gets close to Pynchon's masterpiece 'Gravity's Rainbow', but something is missing; I can't quite tell whether the book simply seems too 1960-ish, or I simply don't get so hooked on metafictional plays any longer, or all of it just seems too real ...
Pynchon is correct in the final line of his press release: 'Let the reader decide, let the reader beware,' or, as Raymond Federman once programmatically named one of his novels: 'Take It or Leave It'.