I already wrote a full review elsewhere. As HTML code is not allowed, I'd advise you to search for "3rd Party Pathfinder Review: Path of Shadows" on Min-Max Boards or Giant in the Playground's respective D20 subforums. There I examine what the book has to offer with an in-depth, chapter-by-chapter analysis.
For an abridged review, this product is well worth your money. It has a little something for every character concept, except perhaps for non-casting martial classes save the Barbarian. The nightblade class is a cool mage-thief type who utilizes a path system for a variety of character concepts based on whether they manipulate shadow for illusion, damaging elements, illusion, the cold powers of death, or the manipulation of shadow energy to create pseudo-physical objects.
Many of the class features, spells, and magic items have good synergy with each other, and the author does a good job at taking care of potential infinite combo cheese. There is shadow magic of many different schools other than just illusion, and evocation-lovers will be pleasantly surprised to find their brand of magic highly represented.
Finally, the artwork is downright beautiful. Path of Shadows is a solid first entry for Ascension Games, and provides a lot of material suitable for many kinds of Pathfinder games.
[5 of 5 Stars!]