As others have mentioned, this film was really done an incredible disservice by the studio's marketing department. It was made to look like a fun, fluffy comedy strictly for girls, but it's so much more than that. I would genuinely place it among the top ten films that have affected me most in my life (alongside such heavyweights as A Streetcar Named Desire and Schindler's List), because it is one of those films that seem to transcend the medium and shoot you directly in the heart, leaving you feeling you've learned something invaluable about the human condition.
At the centre of the film is an astounding performance by Toni Collette - one that stands with the greatest character studies in cinema history (I know I'm using lots of hyperbole, but this film really justifies it!). She has you howling with laughter one moment and then makes your heart ache the next. As if this isn't enough, the supporting cast is also uniformly amazing. Rachel Griffiths is awesome in a very early role (pre-Six Feet Under of course...), and the actors and actresses playing Muriel's family are all brilliant in their own individual ways - especially her father and mother.
This is a film that WILL make you laugh hysterically at times (and will have you quoting the lines for the rest of your life), but it will also make you cry. And after it has finished, you will feel that your life has been immeasurably enriched.
Watch it. Please.