I used to call myself a Satanist.
I don't really anymore.
The main reason I called myself a Satanist was because LaVey's views as he explained them in the Satanic Bible (9th edition) were extraordinarily similar to my own. I already held those very same beliefs and opinions, but I didn't know they had a name. Satanism gave me a way to concisely explain what I think, even if it ultimately confused more people than it helped get the message.
("I'm a Satanist" is a shorter way of saying "I'm an antitheist with a particular dislike of Christianity who also believes that basic human impulse, coupled with mild indulgence to characteristic human selfishness and adherence to a slightly modified Golden Rule, is the sanest, healthiest, and happiest way to live your life, for both yourself and most everyone around you," but only to those who know what Satanism is.)
I left for reasons that include:
-The second half of the Satanic Bible (the part about magic) is hugely hypocritical. It spells out in the first half that the universe is more or less free of supernatural elements and that we should focus on the human being... and then comes out of left field with instructions on how to cast a fireball. What the fuck? The first half mocks everything about a holy ceremony, including the idea that saying some cool-sounding stuff methodically will actually produce any effect, but the second half tells you to do just that. It's ridiculous.
-I'm sick of explaining what a Satanist is. I kind of liked the shock value of it at first, and it was interesting to see some reactions, but overall it's not worth it, especially if you list that on an online profile and a potential employer sees it. Hardly anyone knows what a Satanist actually is, and of those who do, very few give a shit. There are even people (such as my mother) who can't get past the name and will continue to think that it is the actual belief in and worship of the Satan of Christianity no matter how much you explain.
-"Satanism" is also the name of what is basically assbackwards Christianity. These guys, whom I call "Moronic Satanists" as opposed to "LaVeyan Satanists," think that Satan is the true creator being who loves us all and that God is the banished angel, and that God made the Bible to trick humans. At its core, it's Christianity with some names swapped. Moronic Satanists give LaVeyan Satanists a bad rep (not that it could get much worse).
-Satanism attracts the usual pagan/goth/emo crowd who only claim belief because of its shock value, and are not interested in the slightest with LaVey's thoughts.
-Many Satanists are in it for the "free love." Sometimes I think that Satanism is more of a way for all manner of ugly, obese, and generally repulsive people with no saving graces to readily identify themselves, band together out of realization of their permanently lonely condition, and engage in depressing and pathetic sexual practices. I mean, if you're an ugly, repulsive little idiot with no personaity, instead of just cutting yourself and weeping silently while masturbating in the dark like you usually do, why not do it with someone just as digusting and despicable as you are? Both of you know that you would never get any on your own, so why not do each other? It's not with any sort of person you'd like to be around by any means, but something is better than nothing, right?
-The Church of Satan is overrun with Moronic Satanists, and has one as its leader. Even though Dr. LaVey's grandson, Stanton "I shrugged off an anthrax attack like it was a common fuckin' cold" LaVey, is working hard to fix this problem, I don't think it can be easily prevented. Moronic Satanists are too abundant, and I want as little association with them as possible.
-By declaring your Satanism, you should expect the rate of death threats from theists to increase dramatically, as many Christians and Muslims feel that it is their God-given duty to slay demons like you where e'er they're found.
-(For me, anyway) the title "atheist" is far more liberating and independent than "Satanist." When I called myself a Satanist, I felt that I was more an extension of someone else's ideals and that I was just a meager follower.
Those are the reasons I can come up with off the top of my head. There were probably more, but I just don't concern myself with Satanism that much anymore. I do have the benefit of knowing what someone is talking about, and I did find a portion of my own worldview in writing that I can now use as a reference, but I won't associate myself with Satanism unless it is in some unusual circumstance socially beneficial for me to do so.