Well, the first thing you should know was that it was created as part of a bet between 2 science fiction authors, L Ron Hubbard and Robert Heinlein. Apparently, they had a bet between them as to who could found their own unique religion first. Hubbard won, with Scientology. The Heinlein church does exist, it's called the Church of All Worlds, but Scientology is significantly more popular.
Scientology is actually pretty scary. They have been known to go all-out to destroy public figures who go against them. For example, the Cult Awareness Network, a group to help families who had family members join cults, called Scientology the most dangerous cult in America. Scientology ran them bankrupt with a continual barrage of one lawsuit after another, and then showed up at the bankruptcy proceedings to buy the rights to the name and the client list.
It's long, but a Time Magazine exposee is at:
http://www.xenu.net/archive/media/time910605.html
Basically, at the high levels you're taught about the Evil Galactic Overlord Xenu who banished spirits to earth 75 million years ago, etc. (no, I really didn't make that up.) Of course, to reach those levels, you're paying out checks on the order of $30,000 a session...
Hubbard himself spent the last 5-6 years of his life living on a yacht in international waters because he was wanted on a criminal warrant from the IRS for tax evasion and fraud. He died in 86 before he could be brought to trial.
-Arlos