I had the opposite experience with this book than I did with The Professor and the Madman. It was kind of a rambling sort of work without much of an overarching narrative, written on a topic I wasn't all that interested in - but I enjoyed it. Susan Orlean is a good writer, is what it comes down to, and she has a lot of interesting reflections on the material (and a lot of really fantastic turns of phrase - the prose is excellent). The book had just the right amount of introspection, where the personal notes actually helped you see why she found the story interesting, or added some drama (she has to face the swamp again! And she's really scared!) without seeming self-indulgent or narcissistic.
Actually, the most interesting thing about the book, to me, was that more than any other non-fiction I've read, this one gave me a kind of glimpse into how books like this come about. You get the sense that she had read some articles about the court case that the book is ostensibly centered around, and then just sort of got into her car and went to find out more. She wrote a New Yorker article, and then developed it into a book. A nice job, if you can get it...
Though I will say that it occasionally seemed a bit... problematic, the way she wrote about people. I mean, there are some pretty unflattering portrayals in there. I wonder if those people read the book and how they felt. And I guess I wonder whether that should Matter, in some grand sense.
I did not end up with some kind of new fascination with orchids or the people who cultivate them, and to be honest, I also didn't find myself caring all that much about the people being described, but the book was definitely interesting, and merited the full length treatment it got. I wouldn't say it's a must-read, but it's certainly not a waste of time.