Some. If my players come to me with a particular prestige class, I consider adding it to the game. They get massive amounts of benefit of the doubt if they can show me how it fits their ongoing character concept rather than just the nifty toys the class provides, though.
My current campaign has two PCs with prestige classes - a Druid whose concept was built around shapeshifting and the player took the Master of Many Forms class (or whatever that class is called now, I don't remember -- it had a name change between Masters of the Wild and Complete Adventurer), and a Rogue/Sorcerer who uses a heavily modified version of the Assassin Prestige Class the player and I concocted (mostly me) to represent the mystic order he wanted to belong to. None of the other players have shown any interest at all in adding a Prestige Class or even multiclassing.
My current campaign has two PCs with prestige classes - a Druid whose concept was built around shapeshifting and the player took the Master of Many Forms class (or whatever that class is called now, I don't remember -- it had a name change between Masters of the Wild and Complete Adventurer), and a Rogue/Sorcerer who uses a heavily modified version of the Assassin Prestige Class the player and I concocted (mostly me) to represent the mystic order he wanted to belong to. None of the other players have shown any interest at all in adding a Prestige Class or even multiclassing.