I own Ghostwalk (and someday if I have time I'll even review it so it isn't ignored).
The rules for playing "ghosts" are very elegent and mesh seamlessly with the standard D&D rules. Basic idea is you die and come back as a "ghost" (incorporeal outsider). As a ghost you continue along either the Eidolon or Eidoloncer class path (those are the only two options, though it does mention a variant of allowing ghosts to take living classes again). Eidolons gain better ghost powers, Eidoloncers gain a few ghost powers and continue their spellcasting progression. If you're raised later, you convert all ghost class levels into normal class levels. Ghost feats taken aren't lost, but can't be used until you die again.
The setting is very generalized. Manifest itself could be dropped into any other campaign world without problems, save perhaps you might need to rethink the afterlife cosmology of the world a bit. The nations around Manifest seem to be based on a dominant trait - sorcerers, barbarians, rogues - and all vary pretty significantly. Yaun-Ti are the primary villians, followed closely by the undead and followers of Orcus.
The book gives information on how to drop Manifest into Greyhawk and Forgotten Realms, though it is mostly just, "Replace this FR country with this GW country, and replace this GW god with this FR god." The whole thing turns out to be a series of islands in the FR if you do that. One thing it does say, however, is the GW afterlife cosmology only happens withing proximity of Manifest. Mainland FR still functions as in the FRCS.
Something no one has mentioned is it details the setting of the afterlife and rules for adventuring in the afterlife. Neat stuff.
Would I reccommend it? As a stand alone, no, unless you had a homebrew world you could easily drop it into. Someone who is playing in FR and likes to mix things up to make FR seem a little less overdeveloped would get a lot of milage out of it if they base their campaign around Lantan, as that's where SKR suggests putting it.
For a few short gaming sessions, however, Ghostwalk would be good. Have the dwarves that escort bodies to the afterlife hire on the PCs as additional guards of the caravan, then have them run into progressively harder challenges as they draw closer to the Viel of Souls. Not a bad one-shot series.