for those that think the movie is left open to speculation, well you clearly didn't understand the point of the movie or book.
Brett Easton Ellis said himself on 'Charlie Rose' that the point of the satire is that the the yuppies are so self absorbed/shallow/soulless they can't even tell that one of them is literally getting away with murder...The book/movie loses all meaning according to Ellis/Harron if Bateman just imagined it all.
Bateman has killed(hookers/homeless people/animals/Paul Allen) and there are hints throughout the movie that prove that(dry cleaners scene/lawyer confession/Paul's apartment)
But he is having delusions too...As the movie goes on he starts to become more and more unhinged and can't keep his mask, the last part of the movie is his complete 'psychological' breakdown where reality blurs with fantasy.
Harron even blames herself for not making the very end scene more clear, but all 3(harron/Ellis/Bale) discuss this on youtube, there are interviews you can watch, and the DVD commentary makes it very clear that it wasn't all in his head.
The more times you watch the movie the more you see the meaning to the things that have confused you as well as other little things standing out. It's a great black comedic satire and Absolutely hilarious.