The latest C5 is very heavy and consequently rather slow and heavy on fuel.
With conventional suspension it offers nothing that isn't available better elsewhere. With hydropneumatic suspension you get something that rides better than anything else in the class.
The hydraulics on the C5's are a lot less complex than the Xantia than it's predecessors - the brakes and steering are conventional, and the suspension is just an electric pump, control unit, electrical ride height sensors and some pipes. The pumps and control units are reliable, the ride height sensors can go because they have moving parts but they're easy to change and not very expensive.
The other reasons people used to by Citroen's was the diesel engines has gone by the wayside too - you can get them in Ford's now, and many other manufacturers have caught up now.
They make a good cruiser for high mileage motorway work, but the steering and brakes have almost no feel whatsoever so they're no drivers car.