For an excellent introduction to the various kind of self-publishing alternatives, check out the information on the Science Fiction and Fantasy Writers of America (SFWA) website: http://www.sfwa.org/for-authors/writer-beware/pod/
I also offer a briefer overview in my updated information article "Self-Publishing—New Options for Writers."
I always advise my clients to adopt a buyer-beware approach to all kinds of self-publishing services. But, above all, avoid the old-fashioned vanity presses that have repackaged themselves as POD services. In the old days, such outfits would charge a huge fee—often in the range of $15,000 to $20,000—to produce several hundred copies of books that would typically end up gathering dust in the author's basement. This was bad enough, but now some of these outfits are offering POD services that include a "complimentary" set of 25 or so books—for the same outlandish cost that once got you 10 or more times the number of books.
Typically, these vanity publishers offer little or nothing in the way of marketing and distribution. While this may also be true of the newer POD services, the crucial difference to note is that many of the latter services are only charging a fraction of what the vanity publishers insist is a fair fee. Why pay $15,000 when you can get comparable services for $1500?
For more cautionary information about vanity publishers, see this SFWA article:
http://www.sfwa.org/for-authors/writer-beware/vanity/
