James Schellenberg has a decent article up on Strange Horizons entitled "Vanity, DIY, the Multicorp, and You". Basically it takes apart the pros and cons of vanity presses and self-publishing.
My own opinions on both vary. Self-publishing has a time honored tradition in literature, and sometimes it's the only way to get a book out that's ahead of it's time or a bit more provocative than people can handle at the time. But now with printing services like Kinko's making it affordable for every crackpot with an idea, it can be a little suspect unless an author who already has a reputation of some sort is doing it. Vanity presses are only good for people who just want a book to hand out to friends.
My direct experience with both goes back a few years.
For several years I worked at an art museum in D.C. that happened to have a small, in-house library. We'd often get packages of strange books put out by vanity presses addressed to "Library" or "Librarian", and they were almost never related to art and almost always very bizarre. We got a bit of everything: books on Bible Code, UFO's, instructions on killing cows, manuals on fencing. If I didn't take them home, they normally just went in the trash.
My favorite, though, was a really bad book of fiction. It wasn't my favorite because of the book itself, but for the "press" materials that came with it. Often a publisher includes a pamphlet of early reviews or blurbs by big name authors, hoping someone will read it and buy a bunch of copies. This particular book came with a blurb sheet containing praise from the author's mom, a high-school janitor, and a gift shop owner in North Carolina. Although I can't guarantee it, I'm pretty sure the author meant it as a serious project.