The Which? make a yearly car survey. On that survey people in the UK, not only members of Which?, rate all aspects of car ownership of their own car, like reliability satisfaction fuel economy or practicability.
The cars are classified in different classes and in the luxury car class the 9-5 I (1997-2010) appears as the class winner.
This is not much information, it only shows that the 9-5 is the best of its class, well it is also the best overall, at least the one with the highest rating. If you visit the Which? site you can also get some information on the reliability of the car. There you can see that the 9-5 is the third most reliable car in its class, and it is the car with the lowest repair costs at £11o, although Which? says that the 5 series is cheaper with an average of £111.
Talking about the other cars, I don’t know if its a coincidence, but you see too many times the name Vauxhall when you look at the worst of their classes. And regarding the luxury class, it is very enlightening to see the Audi A6 as the worst of its class, although Audi “the Brand” is, when you talk to young professionals. I would say that people set higher expectations on “the Brand” than it can fulfil.