Two words: totally hilarious.
I won't say too much because there's already been so much written about how great and truly funny this movie is, and how supremely hilarious Kristen Wiig is in it. Suffice to say: it's about time! Women are funny and this totally proves it (as if Tina Fey hadn't already done that) - they just need a good vehicle.
I read an article about how the time for women's gross-out comedy had finally arrived, and how previous punters like Cameron Diaz's The Sweetest Thing were just ahead of the market, but I say pooh pooh. That movie was just plain crap with a terrible script and cliched storyline. This one is just flat out funny with no dull bits.
If you haven't already seen it, I highly recommend it.