I see other people are raving about this movie, and I can understand why. It has a wonderful cast (Hugh Grant, Toni Collette, Rachel Weisz, and Nicholas Hoult) who delivers great performances, it's paced well, it is, of course, funny, the scenes are well-chosen, and it's edited just right.
While the book About a Boy by Nick Hornby is a great little gem, turning it into a movie was a different project and could have gone all ways. Besides the script, direction, and performances, I really liked the camera work -- it was in tune with the character of the movie, which, by the way, includes some nice asides. There was also some nice music by Badly Drawn Boy.
Chris Weitz and Paul Weitz directed the movie. Peter Hedges, Chris Weitz, and Paul Weitz adapted Nich Hornby's novel.