Awful movie. Spoiler alert: the main character gets raped at the end by his ex-girlfriend right before the 40 days are up, and it's seriously wrong not only that she gets away with it (and we assume along with thousands of dollars from the bet), but also that he has to apologize to his love interest, and NO ONE in the movie seems to think seriously about it. Ugh. As a female, this really aggravates me, because the whole concept of what rape is is totally lost in this movie. Besides, if men should be held accountable for sexual harassment, assault, rape, etc., women should be too, even if it's comparatively extremely rare. I'm very surprised that I only see a few other reviews mention this.
It's hard to believe that men actually go through something like this when they're sexually frustrated. Sure, my guy friends have told me how often men think about sex, and I get that. But to portray a nice young man like this is rather sad and pathetic, and especially despicable are his roommate and friends. Overall, if you feel like wasting your time, go for it. The actress who plays the character Erica is probably the one redeeming quality, while Hartnett has a few moments. Just be prepared to feel disappointed in how men are portrayed.