To be honest, not very. PrC's get much more use from the DM than the players. But then, there are more NPCs than PCs.
In the current game, there is only one PC with a prestige class: a Ranger/Ghostwalker. However, I've sold the party monk's player on picking up "perfected one" once the party reaches epic levels.
Recent NPCs have included:
Many drow True Necromancers, Pale Masters, and Templars. Coming soon: Drow elf bone archers!
A sorceress/mirror master (helpful NPC)
A female conjurer/summoner (main villain in campaign)
An aristocrat/fighter/blackguard (major villain, recently slain, but will probably be back as a death knight.

)
A bloodless wizard/archmage (which convinced me that archmages are too powerful)
A cleric/summoner servant of the resident tyrant god.
Various cleric/warpriest servants of the resident tyrant god.
The king's fighter/rogue/devoted defender.
A turncoat villain Wizard/Spectral Loremaster