For me, it's been the reliability and the "umph" of a nice V6.
Purchased my 04 in 07 and have done nothing but basic maintenance and repairs. Car has never let me done.
For a sedan it's "zippy" as one of my brother inlaws says. V6 power is enjoyable. Plus I just like driving this car. It's a nice cruiser on the highway. Also gets decent gas milage for a car it's size.
What is it about the Grand Prix that draws you to it?