Hal (Jack Black) and his friend Mauricio (Jason Alexander) spend their lives fixated on the outer beauty of women. They'll go for the ladies that have little in common with them, but are too shallow to understand this.
A chance meeting with life coach Tony Robbins in an elevator enables Hal to see just the inner beauty of women conveyed as their outer beauty as the result of a psychological test. He starts to have more luck with the ladies and falls for Rosemary Shanahan (Gwyneth Paltrow), his boss's daughter.
As his friends believe he is just dating her to gain a promotion, Mauricio decides to track down Tony Robbins in order to reverse the psychology. As the spell is broken, Hal realises that Rosemary is hugely obese and ignores her, but after realising that her inner beauty is perfect enough, Hal realises he wants her back.
Shallow Hal is an intelligent comedy. While at heart it is a rom-com it also conveys a deep moral message about human nature that is often included in films by the Farrelly Brothers (Stuck On You, for example). Unlike most other films of an overly-done genre, Shallow Hal is one that may well be worth watching.
Fundamentally, however, it is flawed. It's implications that all beautiful women are ugly on the inside and all unattractive woman are great on the inside is an irritating and shallow side-effect of an otherwise great film.
Jack Black is funny as always and is aided well by the less well-known Jason Alexander, but the real star of the film is Gwyneth Paltrow whose full range of emotions are on display - even inside the specially-made fat suit for the 'real' character.
Good, but not essential.