Well, all right, this movie isn't falling-down hilariously funny, but it's a far cry above the gross-out "comedies" that are popular at the moment. All too often a movie that's meant to be funny turns out merely crude and vulgar. I, for one, was pleasantly surprised by Shallow Hal.
The plot, as you already know, is about a guy who is completely focussed on women's outward beauty, to the exclusion of all else. He considers himself a playboy, and can't understand why he can't keep a girlfriend. After a mental adjustment from a motivational guru, he begins to see each woman's inner (not outward) beauty. The joke is, he really SEES them as who they are inside... leading to his falling head over heels for a very large woman named Rosy, played by the ultra-slim Gwyneth Paltrow wearing a costume that must've weighed more than she does!
The difference in perception between what he sees and what really is forms the basis for the humour in the movie. Yes, there are "fat jokes" in the movie, but that's to be expected considering the plot. For instance, it wouldn't be nice to make fun of an overweight woman whose chair collapses beneath her at a diner, but when it's a girl who can't possibly weigh 120, the ludicrousness of it catches you off guard. That's the type of humour the movie trades in, so if you think you'll be offended, you probably shouldn't watch.
Overall, the movie's message is good: People deserve to be treated with respect no matter what they look like on the outside; it's what's inside that counts. And there are poignant moments, such as when Hal, in the Pediatric Unit of a hospital, realizes how shallow he really is.
I thought it was a good attempt; it definitely had its amusing moments, and a unique way of making its point.