It's a bit early to tell; I've only had my '03 Buick Park Avenue for three months. But it has performed perfectly in city and highway service, hot and cool weather both. When I had the coolant changed early this month, my regular mechanic reported no leaks, and the coolant level has not dropped even over my 1000-mile road trip to Little Rock. No oil used either (presently on Chevron Supreme 5w30 w/ 12 oz. of Auto-Rx).
I call gas mileage a wonder, now: About 21-22 on regular gas in 75% city driving, and I saw 31.9 on the open road at 70 mph. Incredible for a 3800-lb. car.
My plans are to upgrade the brakes for more stopping power, have the trans fluid and filter changed, and just drive it.