The book is really good, but I think both medium have their good qualities. (Despite a book's detail, you can't really grasp your head around visuals of the situations. Gilliam did a great job with that.) The movie shifts events here and there (it's sorta like an acid-trip of the book) and it had to end it a little short, but overall it captured the spirit of the book.
Of course, the man isn't about doing drugs all day. Though the amount of drugs and things he got away with are staggering, he'd just be another Cheech or Chong or Timothy Leary if thats all he did. He's very good at getting into the depth of a situation, analyzing it, being a part of it, and exploring the reasons why it exists. Hell's Angels is a good example of this, and it's a very good read.
Rosalina: But you didn't.
Robert: But I DON'T.
Rosalina: You sure that's right?
Robert: I was going to HAVE told you they'd come?
Rosalina: No.
Robert: The subjunctive?
Rosalina: That's not the subjunctive.
Robert: I don't think the syntax has been invented yet.
Rosalina: It would have had to have had been.
Robert: Had to have...had...been? That can't be right.