The humor comes from different places. The book's humor is almost entirely internal, derived from Lucy's witty, self-deprecating, and wildly over-analytical narration. The film's humor is more external: physical comedy, snappy dialogue exchanges, and reaction shots.
Both are funny, but they tickle different funny bones. If you love a sardonic, interior voice, the book wins. If you prefer seeing two people pratfall around an office with paint, the movie delivers. It's a classic case of adapting the plot but having to reinvent the comic engine.