Wasn't bad. As a time saver, I would almost recommend reading it in reverse.
This book contains three parts. The first part is 3/4 of the book or so "The Atrocity Archive". The second part is a short story in the same world with the same character within "Atrocity" called "The concrete Jungle". The last part is a reflection by Charles Stross about books of this nature.
So the last part of this books waxes philosophical and otherwise about the nature of a book like this in the world. It is probably what you should read first. If what is described within sounds interesting to you, then read from the beginning. If it sounds awful, then read something else.
If, however, you aren't sure, then I would recommend reading the second story "The Concrete Jungle" first. Did you like that? Then you will probably like the first (and much longer story) "The Atrocity Archive". There, I made efficient use of your time.
Personally, this really wasn't my thing. If felt too much like Star Trek dilithium-crystal-deus-ex-machina kind of mumbo jumbo to me. To be fair, I don't enjoy horror (maybe it is a lack of imagination), so I may have just been skimming when I was supposed to be enthralled.