Fascinating, head-shaking, and very readable, in a nutshell. Beyond that, the author seems as though he's bending over backwards to be even-handed in this book, even though I'm sure he's been on the receiving end of what the church of Scientology dishes out to anyone who says anything negative about them (stalking, harassment, slander, etc).
As has been mentioned, the book is an expansion of Wright's article on Paul Haggis (former Scientologist) in the New Yorker, so the evidence leans on Haggis's interviews a lot. But the background on Hubbard is truly fascinating. I had always pictured him as kind of laughing in his sleeve about all the crazy stuff he came up with as a cosmology for the Scientologists, as he milked more and more money out of the believers. But according to Wright's read of Hubbard, he (Hubbard) was also a true believer, at least in the later part of his life.
I found Wright's analysis of church vs. cult very interesting and even-handed too, as well as his discussion of whether the organization engages in human trafficking with its Sea Org members (the somewhat-sequestered clergy who do all the grunt work).
This is, of course, where Scientology's greatest criticism is: the abusive practices against its most dedicated members, and the resulting difficulty in "escaping" the church, which is what leads journalists to use the word "cult" in describing the organization.