Wrong actors
Wrong actors for this film. The guy who played Neil Cassady couldn't have been further off in character. Cassady had a mid-West twang, the actor didn't look anything like him, didn't capture Cassady's charm and charisma at all. Viggo Mortensen played a good Bill Burroughs. The guy who played Kerouac was okay, but not great. The Ginsburg character was good in physical likeness but didn't get Ginsburg's accent either or charisma.
The script was pretty good. But On the Road is a romantic novel about rediscovering America as a young man and breaking out of conformity after a long period of calamities (WWII and the Great Depression). The film would have done better of it caught more romantic images of the American landscape in the style of The Straight Story by David Lynch. The drug scenes would have been better if we could experience the drug experiences first person instead of second person. In other words, the drug experiences should have been filmed more like Fear and Loathing. The way On the Road did it made us observers which didn't capture the romance and excitement of the drug experiences. We just sat back and watched the effects on Kerouac and Cassady, which didn't really deliver effectively.
The sex/romantic scenes were done well, particularly when Kerouac was picking cotton. That was right from the book and cast well.
All-in-all slightly disappointing.