Although the Scandanavian cubist design was starting to get a bit old by 1998, one thing that they got spot on was the interior--it is absolutely fantastic. The leather seats with suede inserts completely coddle their occupants. And the tasteful, real(?) wood accents would not look out of place in a car costing much, much more. And while I was a bit uncertain about the Volvo 24-valve V-6 engine, they had gotten all the kinks worked out by the '98 model year, and it's nice having some power in a Volvo. The car is barely middle-aged in terms of mileage, and I'm sure I'll tire of it long before it's in need of retirement. The S90 is the ultimate old-school, rear-wheel-drive Volvo.
Definitely the interior. Build quality is amazingly solid. Side-impact airbags in a 1998 sedan! Great value for the money. I paid the original owner $4k for a 142k mile example back in 2010, and spent $1k on a new timing belt, water pump, and tune-up. If you're looking for a vehicle that represents the pinnacle of Volvo's rear-wheel drive era, the S90 is the car for you.
Repairs can be expensive. Headlamp wipers are useless. You better like sharp angles.