I've had 5 of them over the years. First was a 2000, then I had two 2004s. Currently I have a 2008 that I daily and I also have a 2010 that I just picked up. They're great cars and being that they're former fleet vehicles, they're typically well maintained.
One thing to keep in mind is the coils start failing around 100K miles, so if it has the original COPs, you'll want to have a couple on hand. If the car develops a miss out of the blue, it's probably a failed coil.
Another thing to look out for is the tires. Every cop car that I've bought came with shit tires because the fleet managers will keep the tires to put on other vehicles if they have good tread and put shit tires on before getting rid of them. They're always out of balance and hardly have any tread left.
For being as big and heavy as they are, they do have a good amount of power. They're fun to drive and make great family/travel cars. I can stuff the three kids in the back, the wife up front, and fill the trunk with luggage.... and still have room to spare.
Some notes:
I wouldn't ever bother buying a pre-2004 model because they made a lot of positive changes during the 2003 run.
Although not really a big deal, the 2006 and up models have a newer gauge cluster with a tach that looks much nicer than the previous models in my opinion.
Beginning in 2008, the Crown Victoria was sold as a fleet-only vehicle and most of them did not come with cruise control as a standard option. So if you're looking at a 2008 thru 2011 model, chances are it won't have cruise unless the agency that purchased it new specifically asked for cruise... which most did not. Cruise can be added if you buy the steering wheel that has the buttons on it, but you'll have to take it to a Ford dealer to have the options enabled in the PCM. It's not plug and play.
Also, the majority of late model cars came with "night mode" enabled by default in which the dome light does NOT come on when the door(s) are opened. This was done so the police could enter/exit the vehicles without the lights coming on. It's a minor annoyance and you can have the "night mode" crap disabled at the dealership also.
Flex-fuel became optional in 2006 and standard in 2008. So if you are looking at a 2008+ model, you may be tempted to run e85 in it, especially since it's cheaper than unleaded in most areas, but a lot of people that I've talked to over the years advise not to do that and they say it makes the car run like shit. They don't need mid grade or premium either. Just stick to regular unleaded.
