BALANCE:
The class is pretty powerful, and that's quite fine, since it's a PrCl. Compared for example with the Rogue (same proficiencies, same BAB, same ST, uncanny dodge), it loses 2 sp/level and Sneak Attack for many more special abilities than the Rogue class.
FEATURES:
The main and most powerful abilities are new and unique for the SD: Hide in Plain Sight, Summon Shadow and Shadow Jump can be very effective and creatively combined with the base class abilities, eventually spells.
Darkvision, although might be redundant with the racial ability, is also very welcome, but basically there is nothing that a PC may find unuseful from this class' special abilities.
FLAVOR/RP:
I think the concept of this PrCl is one of the most original I have seen, at least for non-spellcasting classes. It is a very rogueish character with mastery in shadow-related but very different things and for very different purposes (defense, backstabbing, stalking, travelling/teleportation, companion/battle support).
It looks so special that I would rather tie the PrCl to an organization, such as a guild: to leave it easily open for everyone would spoil a lot of flavor of it, IMHO.
REQUIREMENTS:
Not very hard to qualify for, the requirements simply reflect the base of the class to be in agility and stealth. A good thing that no requirement exclude any class to qualify for the SD.
It seems to me that the Perform requirement was deliberately set lower than the other not to become an extra delay for Rangers to qualify.
DESIGN:
Some gaming groups have had problems with a couple of abilities being quite easy to abuse, but I think a prepared DM can handle them. Apart from that, the overall design is very inspired and careful.
I think a d6 (or even d4) for HD would have been more appropriate, but it's a minor complaint.
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OVERALL: 10