Here is my personal take on a few American Psycho myths and longstanding controversies (please note that these theories are solely based on my experience reading the book and watching the movie, both a long time ago).
Oh yea, and there are spoilers too (duh!), so do not read further if you've not read the book and intend on doing so.
1. American Psycho/Bret Easton Ellis is a misogynist book/writer
This has been by far the most important issue with the book upon its release: many feminist associations directly attacked the book for its content. NOW especially led boycotts and organized protest against the book's "glorification of misogyny"...
I must say I did not even considered that aspect when I first read it: it should be rather obvious to anybody with a bit of critical sense, that the thoughts and acts of a novel character do not necessarily mirror the ideas of its author or the message he wants to convey, quite the opposite actually. Even though Bateman is a misogynist killer, brutally murdering women in all sorts of gruesome ways, that hardly makes the book a misogynist one... It should be fairly easy to discern denunciation from justification, I think this is a clear example.
Of course, for the sake of the novel, Ellis has to make the main character somewhat appealing, or at least give him enough charisma that the reader can feel some kind of limited empathy toward him. This multi-dimensionality is what is cruelly lacking from the movie: afraid to seem like they are condoning the acts, the directors safely stepped back and gave it more of a third-person point of view. In both case, it is not like Bateman's vacuous and ridiculous life is pictured under a very positive light overall: he is a bad guy, doing mean things, no question about that. Therefore his acts have to be seen as an illustration of evil, not a glorification... No reason to think Easton intended it otherwise.
Now I also read in some article that feminist organizations actually reproached Ellis his depiction of women, which makes more sense from an intellectual standpoint. Indeed, most women characters are either money-hungry bourgeois bimbos or peroxide blond airheads. But on the other hand, the only redeemable character in the whole story is also a woman: Bateman's secretary with her honesty, sweetness and simplicity is the only one who even manages to escape her boss's senseless murderous impulses.
It is worth noting that the producers of the movie version were obviously scared senseless by the possibility of such an attack against the movie and therefore picked two strong radical feminist women to write the scenario and direct the movie. This smart move avoided them most of the criticism the book got, but also made for a heavily reworded message and one-track tone: Bateman's strange and complex character is replaced by a unilaterally ridiculous buffoon and any of the original creepiness in the book is channeled into easily comical kitsch situations. This make, in my opinion, for a rather interesting movie, but with none of the breakthrough quality of the original material (I am aware, though, that a literal adaptation in today's mainstream film industry was seriously out of the question).
2. The whole book and everything described by Patrick Bateman is all just a dream
My opinion, but: No.
Among my biggest issues with the movie (which was, in other aspects not such a bad adaptation) was the direct implication toward the end that everything we had just seen was the disturbing, yet harmless, product of a slightly psychotic yuppie's imagination: due to excessive drug use, mild predisposition and too much time to fantasize...
Well, this "t'was all just a dream" theory is never seriously alluded to in the book. To me, one of the main points in the book was precisely to show you how easily Bateman's acts could go unnoticed in modern society. The fact that he keeps dropping hints, makes a full confession over the phone or that his victims seem to reappear mysteriously in other parts of the world is just a statement about the egotistical blindness of these people: they cannot bring themselves to really look at anybody/anything else than themselves. They see Bateman and do not try for one second to go past his mask, even when he provocatively gives them a clear glimpse of his sick mind. It is also a society where every characters are more or less identical suit-wearing yuppies: nobody ever seem to get anybody's name right and employees of a same company are constantly being mistaken for each other.
In such a context, it is not surprising to see their disappearance going mostly unnoticed (as a point of reference, we are given the case of Tim Price, whose sudden unexplained absence barely raises a comment from his "friends").
It is clear nonetheless that Bateman's psychosis steadily increases during the whole movie. The state of decay of his apartment, his bouts of remorseful confessions and his increasingly careless killing fashion ought to get him eventually caught... In typical two-bit folk criminology, he desperately craves attention. And maybe that's what the final "this is not an exit" message is all about: he won't be getting any by acting that way, people just don't care enough. Or maybe it simply refers to the fact he will not eternally get away with it.
The real estate agent's attitude at Owen's former apartment goes this way too: she is obviously spooked by his mildly strange demeanour and white lies, indicating some really nasty stuff is linked to this apartment (although the family or the estate agency obviously tried to avoid unnecessary publicity on this), but she prefers not to make any waves and simply ask him to leave and not to cause any "trouble". To be fair, she might also be frightened for her life, but she does not seem to think he is truly the murderer either: whatever his connection to the affair might be and despite the possibility he might be directly implicated, she'd rather see him go discreetly than see him arrested.
3. This book is a caricature of the whole 80's yuppie subculture
Endless discussions about whether this is all a depiction of Reagan 80's, early 90's, New York bourgeoisie, finance shark yuppies, or even a prophetic vision of the late 90's dot.com era.
In the end, there is obviously some strong tie-ins between the ultra-capitalistic lifestyle of other "normal" characters and Bateman's specialization in physical violence. It is pointed numerous times that his acts are a mere metaphor of this lifestyle, the ultimate concretization of the Financial world "kill or be killed" ethos (cf. the famous "mergers and acquisitions/murder and executions" line). These ideas are not excessively new ones, not the appanage of a specific era, and they have not disappeared over the past decade. Capitalistic and socialistic trends in societies tend to interweave and surface alternatively, depending on lots of economical and sociological factors.
No matter what, 80's culture is at the heart of the story: every aspects of this era (coke, 80's fashion designers, Republican politics...) are present in the plot, whole chapters are devoted to a semi-serious analysis of a few seminal eighties acts (Whitney Houston, Huey Lewis and the News etc). All the cultural hints and name-dropping (most famous place/people names mentioned in the book were real, others were thinly disguised allusions to existing ones) firmly point to a specific class of people (wealthy yuppies working in finance corporations) in a specific location (Manhattan) during a specific era (the roaring eighties)... That being said, it certainly can be applied a lot of other contexts (although with much less realism, in my opinion).
I think this book does not deserve to be vilified as it was, at least not for the reasons I have seen so far. It is equally unfair to treat its theme lightly by making it mere imaginary productions of a sick mind, while overlooking the strong social criticism.
Whether you like it or not (I cannot say I truly enjoyed it, although there was no way I could drop the book once I started it), this book is definitely an essential part or contemporary literature, both in terms of stylistic innovations, extreme content and vitriolic satire of late 20th century culture.