Great question! I get that question often as to why I have owned 6 Grand Prix's (that's the only kind of car I have owned since 1999) I currently have 4, all 6th Generation. All are 2-doors. I still like the looks of the car, even though everyone says the body style is dated. I agree to disagree. I hate the exteriors of the modern cars, everything from the Impala, Malibu, Cruze. I don't care for anything Ford or Fiat Chrysler. And forget the imports. What is left? I was sold on the 2-door GP since the late 90's. I'm not changing now.
Most of the reasons have already been mentioned above. I find my Grand Prix's to be quite reliable. I have only had 1 breakdown in the past 15 years, which was on my 40th anniversary that had the tensioner pulley bearings go out.
To me, the GP is easy to repair (I do all my own repairs and upgrades myself), parts cost is low and made easier since I have all the same generation of GP, insurance cost is low, lots of factory options, lots of modified parts available, and the main thing is all my GP's are fun to drive. If I did not believe that, I would sell all of them tomorrow.
Now that I am writing all the good things about the GP, one other important item is that it is great in winter weather.
I am truly amazed how the platform has lasted as long as it has. About 2007, I predicted that the end was near for the 6th Generation GP, but now, they seem more popular than ever. All one has to do is attend the W-body/3800 Gathering in Detroit and can see that for oneself.
Hell, I thought about 7 years ago that I would be the only one driving "old" model GP's.
I am still happy as can be when I take one of my GP's to a car show, or on a trip. I am amazed at how many people stop to chat and admire one of my GP's. Most state that they also owned a GP at one time and loved the car. They claim they wish they would have kept the GP, but decided not to for whatever reason.
I live in western Pennsylvania, and there were many Pontiac dealerships. In the area I live, some dealers were only 10-15 miles apart from each other.
A salesman from a former Pontiac dealership said he used to love the days of the stylish Pontiac Grand Prix and Bonneville in the late 90's and early 2000's. He said when his dealership got a shipment of GP's (and Bonnevilles) all would be sold in a few days. And buyers were stopping to ask when more would be on the way. This salesman said after then, sales fell way off, and then Pontiac was gone.