So, everyone already knows the story well, no; chances are, if you're like me, you read the book and yet remember almost nothing of the story. The book burns through its shreds of storyline as if they were just tinder for the blaze of its energy; the real fuel is the pacing, even with all its redundancy. It's the momentum that sucks us into the breathless chaos of Kerouac's world. We come away impressed by the energy, not the content.
Film could certainly have been used to amplify this effect, but this is not that film. Instead, we have a more conventional treatment, focusing on character development. It's a nice production, with an attractive cast. But the story comes at us very differently from the book experience. The manuscript has been rewritten to add breathing space and objectivity. We see Sal Paradise, only half-formed at the start of the story, pull himself together to become a serious writer. We see the endlessly exuberant Dean Moriarity ultimately coming to grips with the progressive self- destruction attributable to his amorality, and suffering. This might be a fair reading of Kerouac's ultimate feelings about that part of his life, but it's not the feeling that Kerouac shares with us in the book. We have lost our innocence; our last chance to revisit it, even for a few hours, is taken away.
I'm not going to rage against this re-conception of the story, though, because it makes other changes from the book that might be improvements. Several episodes that were censored from the book are restored in the film. (Some discussion of this at http://www.univie.ac.at/Anglistik/easyrider/data/BeatEros.htm). So the movie is more historically accurate, and far more sexually explicit than the book. (That could also explain its delayed US release). In one poignant scene, Carlos Marx (Allen Ginsberg) is whining to Sal about how vulnerable he feels due to his poorly-returned love for Dean. To the best of my recollection, that conversation was not in the book (please tell me if you believe otherwise), but was expressed in a private letter from Ginsberg to Kerouac many years after the fact. This kind of thing changes the emotional flow of the story, certainly, but it adds depth, too.
Few of us will actually suffer nostalgia for the gritty overindulgences of the Beats. But remember, this came at a time when society was absolutely saturated with the message that everyone should be "normal," safe, predictable. Without the tiny minority of Beats attacking that message, and specifically without On The Road to chronicle that attack, the cultural revolution of the 1960's would have been even more difficult than it was, and perhaps less effective. Good, bad, or ugly, we must embrace this story.