I just passed the 8 year mark of my Grand Prix GT sedan purchase and am a few hundred miles away from 110k. It's been a fun car to drive over the years. I've had my share of issues, but nothing too major. The alternator failed around 90,000 miles. The power drivers window quit working around 100k. The A/C needed recharged earlier this summer ($180) The LED display on the radio quit working between 90-100k. The car still gets around 19-20mpg in day-to-day city driving but I often get over 30 mpg on road trips - and that's with a family of 5 and a full trunk. I just test drove a Sienna but really don't know if I want to give up the power/handling of the Grand Prix.
Lots of power, good handling. The Red console lights look very nice at night. The power drivers seat with lumbar support has been very handy on long road trips. The 8 speaker audio system has been enjoyable.
Better city mileage on the 3800 engine would have been nice. The highway mileage has been outstanding IMO. Better electrical engineering/reliability.