Like a lot of newer D&D stuff, Ghostwalk's application in games is up to the player. As a standalone game it is fair, though its ties into the D&D cosmology poses more questions than it answers.
It is best used as a meta-setting the same way that Planescape or Spelljammer are used. Making a few assumptions helps this game truly shine and answer some of the great unanswered questions of the D&D Cosmology:
Assumption 1-
Each prime has a "ghostwalk" leading to a distinct True Afterlife maintained by the gods of their world.
Assumption 2-
Souls that "willingly discorporate" in the True Afterlife are lead into the Well of Souls and out into the Planes.
Assumption 3-
Demihumans everywhere go through the Etherial voyage mentioned in the book, however in most primes the etherial boundary is too thick for ghosts to manifest through sheer will--so they either remain stranded or embrace the seductive power of the Negative Energy Plane into themselves, becoming corrupted into undead forever, even in the True Afterlife.
This resolves a lot of questions. What's the difference between the dead and undead and why do the undead have a connection to the Negative Energy Plane?
With infinite Prime planes, why isn't the flood of souls to the Outer planes neverending?
How do dead folks get to the planes?
If Ghostwalk is it's own cosmology, why does it mention the planes and intrinsically D&D characters like Orcus?
But yeah, Ghostwalk is a lot of fun. The entries on the various cultures are awesome, the listing on the city of Manifest is great. It contains the origins of the Yuan-Ti, which by itself is reason enough to get the book. The authors really put did put some quality effort into this--the writing is very polished and the artwork is very impressive on the whole. The absence of a name for the game world is very strange, as I would like at least a suggestion of what to call the world where this all happens, but otherwise an excellent setting.