I've heard bad things about ex police cars, from ex police officers. They have said they wouldn't touch them as they are heavily abused.
Yes they are serviced regularly, and parts replaced but only when noticeably worn or damaged. They also said the parts that haven't been replaced are likely to be on their last legs as well as the strain that has been placed on the chassis and body over its time in the force.
The other thing, as Lenny said is, there are likely to be alot of holes where parts have been removed, such as the lights on the outside and the radios and light switches etc on the inside.