Have not read the book, so I can't compare their quality, but thought it was a very good movie. Not a masterpiece by any stretch of the imagination, but just a fun (very gay) romance with some funny and sincere scenes. The kind of thing that helped my with coming to terms with my own sexuality.
As far as criticism goes, the acting was a bit questionable at times, and the script a tad unnatural at some points, but there are also scenes with some rather good acting and wonderful writing. The film's flaws are obvious, but so are its victories and personally I feel the latter outshines the former.