I was prepared to hate this movie, when I first rented it. It was a 'curiosity pick': I liked the title, I didn't have anything else in mind, and my thought was, of course, "Bill Murray!?"
It turned out to be one of my all-time favorite movies in the 'character-transformation' genre.
It's a compelling story of choices in life and how those choices affect or disaffect perceptions of that life. Murray is perfection in this role, because we seem him change from something we recognize to something quite different. It is a palpable and comprehensible transformation - the movie draws us in, it allows us to change with him.
The rest of the characters are well cast and provide definitive counterpoint to the protagonist - the bon-vivant aristocratic uncle Elliot who simply lives his life appreciatively, the unthinking and manipulative Isabel concerned only with her own comforts and social standing, the uncertain Sophie that allows her uncertainty to trap and destroy her, the practical yet contemplative Raaz who challenges Darrell's notion of things, constructively, and leads him to further his quest. Good character development, all around.
Finally, I was impressed with the faithfulness to the book. It's difficult for a movie to be that, and still be an watchable movie.