well, I think the big problem I had with that movie is the glossing over of the "american dream" theme from the book. the first time i read the book, it was that search for the possibly non existant american dream that struck me the most. the taco hut episode (part 2, chapter 9: Breakdown on Paradise Blvd.) is totally missing. that scene is crucial to the book (and should be to the film).
to find that the american dream was "a huge slab of cracked, scorched concrete in a vacant lot of tall weeds. The owner of a gas station across the road said the place had 'burned down about three years ago.'", is a sobering moment in an otherwise besotted book. that burnt down club is the promise of the '60s hippie culture (if it was promising) being thrown onto Nixon's hearth.
without the search of the American Dream, the book is funny as all get out and high speed madness and even counter counter culture, but has little point.
I felt that way about the movie. I liked it, but was let down.