I liked Brent Weeks' Night Angel trilogy, and I've been interested to know more about how it all started. The Perfect Shadow provides a very small taste of the answer.
The Perfect Shadow reads more as a collection of scenes than as a short story. In fact, one might suspect it of being composed largely of the author's background character sketches, loosely tied together. It provides the answer to why Acaelus/Durzo became a wetboy (assassin), but in a strangely unsatisfying way. While the story is told from Acaelus' point of view, there's very little introspection. It's a classic case of telling and not showing, so that while we know he's upset about losing wife after wife to age and violence, we don't really feel what he's feeling.
All in all, it's a mildly interesting addition to the series, and only for fans. While chronologically first, it should definitely be read last. Without knowledge of the main trilogy, this story will both confuse readers and spoil later revelations.