I've read most of the SSS sourcebooks, and in my opinion this is - BY FAR - the best DD3E product they have released. I do not use the Scarred Land campaign world myself, but I do plan on integrating this into my campaign world immediately.
Hollowfaust continues the dark theme of the Scarred Lands campaign world, but flips the stereotype of the evil Necromancer on its ear, and turns what could be a very predictable city into a unique supplement. The idea of Necromancers as somewhat benign scholars is as intriguing as it is bizarre, as are all of the attendent situations, such as undead acting as both unskilled labor and the city "protectors".
The treatment of the city, its history, adventure hooks and resident personages is detailed enough to make the city playable, but not so detailed as to overwhelm or restrict.
The prestige classes are a useful continuation to those provided in Relics and Rituals and Tome of Blood, and can provide depth to a campaign world, although for the most part I would not use them as PC classes.
All in all, I enjoyed the City of Necromancers immensely and think that anyone could use this in their world with a minimum of hassle and maximum results.