I have owned 3 GSs over the last 6 years and the 98 is the one I currently drive. This car is smooth, well built and a pleasure to drive. Buying it used is easy, because it's a Lexus. No unscheduled repairs (I've put 55k mls on this one and 65k mls on another)! Almost same interior as the LS with better mileage. Better quality feel than the ES. It is expensive to service, but far less than a BMW or Mercedes would be, and if you find a good independent repair shop you're a lot better off. I highly recommend this car.
Really great ride and luxury feel. The Nakamichi sound system is wonderful.
If you have one with low profile tires try replacing with the Bridgestone Potenza RE950. It's smooth and will last 2.5 times the stock tires.