There are three kinds of comedy. Comedies that make you smile (Sixteen Candles), comedies that make you laugh (Airplane), and comedies that cause you to nearly asphyxiate yourself from laughing so hard. Mary falls in the the 3rd category.This is the Farrely Brothers at their funniest. The idea is simple, a guy (Ben Stiller) who had a crush on a woman (Carmen Diaz) he knew since high school and continued to yearn for her privately, finally getting an opportunity to try to reconnect what never got going. This is something almost everyone can identify with.
When you watch it you just know how it's gonna end, and so the focus is not on the plot but the actual happenings in the movie, and there's a lot. First, the movie is funny, with its many gags that just keeps on coming (a Farrelly Bros. tradition). They might not be in everyone's taste, but at least they aren't forced or out of place.
Second, the people seem real enough to be in the situations they brought themselves into (kudos for the great acting, though it could be better). Third, Cameron Diaz is adorable, beautiful, and a marvel to look at unlike in failed ventures such as The Sweetest Thing. Plus, she got the job carrying the whole movie and she accomplished that ever so gracefully. You just can't help but fall in love with her too, along with every guy in the film.