As I've explained in past blog posts, much of the theology I grew up with came from the Jehovah's Witnesses, and they make a big deal out of old Lucifer and his band of demons. See, they're out there, and they're always looking for some kind of way to influence you, but if you just call upon God, then Jehovah/Yahweh/Jesus will send them packing. As a result, I spent many nights under my sheets saying "Jehovah" a few extra times just to make sure that all the demons were gone.
(Side note - I sometimes get grief from my fellow nonbelievers for supposedly wanting to take away the "comfort" of religion from people. That bears another blog entry on its own, but let me just say this - not every religious idea brings "comfort". In fact, some of them bring quite the opposite of that.)
Things are different now, obviously. I attended a church service a few years ago, and the pastor mentioned about how some of the folks there were currently undergoing struggles with Satan. I successfully managed to not roll my eyes, but it took some effort. Sorry, but it just seems funny to me now. I also find it oddly amusing when people on Facebook refer to him as "the enemy" as though he's flippin' Voldemort or something.
With all that said though, while I obviously don't believe in a literal Satan, I do believe that there is a lot of power in the idea of Satan.
No, I don't think that he works very well in the context of Christianity for several reasons. For starters, he's a mish-mash of concepts that don't quite come together. The whole thing with him being the serpent in the Garden of Eden, for starters, is a total retcon. Yeah, he's in the book of Job as well, but go talk to a Jewish friend - he's hardly the evil being who Christians make him out to be. He's more like an employee of Yahweh, tasked with bringing up challenges. Obviously, he's an evolved concept once he gets into the Christian scriptures, perhaps having an influence from Angra Mainyu of Zoroastrianism. (And let's not forget that his look is derived from a satyr, who represented sexual pleasure (often sporting an enormous phallus) that the pagan Greeks prized so much and the Christians poo-pooed. Even within the context of Christian tradition though, his role has changed. According to the University of Missouri-Kansas City School of Law's Douglas O. Linder, it wasn't until the 13th Century that the role of Satan went from that of a "mischievous spoiler" to the Prince of Darkness we all know and love today.
Speaking of Satan being a willing participant though, that's exactly how he works when it comes to propagating Christianity. (Or Islam, as I know that he plays a role in that religion as well.) For many Christians, when they come to start doubting their faith, Satan is a convenient entity to blame. In other words, it's a good way to stop questioning, as the very idea of it is evil, so why would you even want to go down that road?
If such an evil entity exists, and the only thing that can protect you from it is God, then why take chances on abandoning the belief that's protecting you from such a malevolent force? This is why many Christians will insist that there's a Satan who's a very real being indeed. I know that I used to be one of them. I would argue with people who claimed to believe in God but didn't believe in Satan. I remember thinking that the "The greatest deception of Satan was convincing people he doesn't exist" argument was really clever. (If you still think it's clever, try replacing "Satan" with "Darth Vader" and realize that it works just about the same.)
In an odd sort of way, I kind of hate Satan more now than I ever did when I actually believed in him. Obviously, I don't hate the being, but I hate the concept of him, as it's obviously designed to prevent people from thinking.
(And yes, I know that some Christians will protest and say that their faith allows them to question. And what are they told to do when they question? Go pray about it. In other words, go reconfirm your bias.)