A lot of the problems with the 2000+ bonnevilles are shared with the 97+ GPs...
GPs are known for BAD build quality after they get a lot of miles (vibrations on door panels) My roomates 2000 GTP is fine though, not a rattle or anything, both of my bonnevilles have more rattles than his GP
Headlights fail (fog or the front plastic actually falls off)
Window Motors fail
Oil Senders fail like every other 3800 out there
Fuel Senders fail just like almost every gm car
Leaks around doors, sunroof leaks
transmissions fail early
Upper Intake Manifold fails due to EGR placement problem allowing coolant into motor
Lower Intake Manifold Gaskets fail allowing coolant into motor
The brakes fail (rotors not big enough, they warp..just like bonnevilles and most GMs)
Other than that they are solid. 3800s last forever if maintained.