In a recent post by Kevin Drum - in which he listed 5 books that he hadn't been able to finish - many of the commenters mentioned Gravity's Rainbow as something they couldn't finish, a doorstop, impossible to read, etc.
I recently ran across a copy of Bookforum that featured a bunch of writers' take on Pynchon, lo these many years after he shook everything up with Gravity's Rainbow. Reading their comments, I was reminded how shattered I was by reading the book. It, more than any other book I ever read with the possible exception of Time Out of Joint by PK Dick, actually changed my brain. I mean, it was like LSD in the sense that "you'd never be the way you were before." You couldn't go back, you were changed forever.

I was so taken by it that I built rockets - here's a bad foto of my 2-stage V2 rocket - painted black - my schwartzraketen - fired over the houses of Anchorage AK in the mid 70s. A thing of beauty. No one else was ever the same, either.