Update: I've toned this down and made it as objective as possible, as I felt I was unfair and perhaps intolerant in the original version.
I remember watching a documentary a few years ago on Scientology, where they attempted to show it up as a money-making scam, and made some convincing arguments, backed-up by evidence. I've never been able to understand its popularity, as L. Ron Hubbard (its originator) was a second-rate science fiction writer at best. I remember reading some critique of his ficion (maybe in Trillion Year Spree by Brian Aldiss?) which pointed out that Mr. Hubbard was an old school sf writer, who got paid by the line: therefore he wrote lots of very short sentences. Bad ones, at that. (By the way, I have attempted reading some of his stuff, and it was so truly awful I gave up.)
Today, I was browsing wikipedia to find out about juche, which Nicola mentioned and we'd never heard of (it's not a religion, as such, but the state ideology of North Korea). While I was there, I looked up Scientology, which Nicola also mentioned. I found a few things which further confirmed my views of Scientology:
Looking objectively at it, maybe few of these ideas are any crazier than Christianity, those of Jehovah's Witnesses, etc.. I also feel uncomfortable criticising other people's beliefs: it would be nice to be better informed about Scientology. However, it's difficult to get at the core texts as they are copyrighted and actively protected; to get at the meat of their ideas you have to pay money; and I don't have the time or the inclination to visit one of their churches, as the ideas seem unbelievable to me (in the same way that those of Christianity, Islam, Judaism etc., in fact any organised religion, do). As Hubbard himself stated in a 1952 lecture: he's "trying to tell you a fairytale" (5203C04B, 1952 Phoenix lecture 2101).
I should qualify all this by saying that, while I am not religious, I defend the rights of anyone else to practice any religion they choose. The thing I'm struggling with at the moment is how far the rights of any religion should be supported when they start to infringe on the freedom of others. I think there is a strong case, supportable by evidence from ex-Scientologists, to show that Scientology (in general, and at its heart) goes too far in terms of extremism, exploitation and intolerance. (Having said that, see the comments for a response from a practicing Scientologist who is actually fairly reasonable.)