Dude, it rocks. I've been slowly introducing things from it as the PCs have come across Mind Flayers. The book is a good read and takes one of the most feared creatures and truely makes them Scarey!!
Picking up and using this book in your campaign will give your players a reason to completely and utterly fear mind flayers. One of Paradigm's best books to date in my opinion.
I'm sure Hound can share the exact details, but it basically runs: the book was in the production pipe when the final SRD came out without mind flayers, and they had to get permissions to finish it.
At any rate, it is a nice, well written book with some interesting background and options. The only thing I could fault is that perhaps the Lovecraftian footprints show a little too clearly in places.
To everyone who likes this book, what specifically do you like? Rather than reading general praises, I'm hoping to get an idea of which specific features/ideas/whatever you got the most mileage out of. Thanks.