I've mentioned it multiple times before, my immediate family is a "Volvo family," among the multiple XC90s, 850, S70, S40, V70, 242, and a recently acquired 760 parts car, dad had an S80 T6. Dad still talks fondly about that car, even though he put a transmission in it.
Comfortable, GT-esque, smooth ride, good road manners, and that T6 was deceptively quick. Like, I just pulled out of the driveway and all of the sudden I'm doing 90 MPH, holy crap, how did that happen?
The T6 issue has already been covered, they eat the trans, learned it, lived it. Otherwise mechanically fairly reliable.
I've described them as having a computer to control everything but the floor mats, towards the end of dad's S80's life, it had a warning message on the display, for all kinds of stuff, but it kept going. The interior had plenty of squeaks and rattles, but the seats and touch points all held up well, which is more than can be said for any GM product made in the past 20 years.
My opinion, and most other Volvo enthusiasts, avoid the T6, but I'd maybe avoid the S89 all together. Instead, maybe get a S60, or if they're into wagons (who isn't) a V70 or V60?