I've worked in Volkswagen dealership service departments on/off over a twenty-five+ year period and can speak about the vehicle from a service aspect. I do agree that a Phaeton is an extremely nice car, the chassis is unlike anything else in its price class and its a fine road car especially with the premium package of reclining rear seats, (2) people, massage functions multi-zone climate control and more. For a used purchase, you get a lot of equipment; its a luxury car all the way with a VW badge; needed either an Audi or some other badge on the hood.
A little history on the car to explain the service quirks on the car: originally VW appointed special dealerships and had specially trained techs, service advisors and even parts people to handle this car. A special hot line for technical issues and even parts had special priority for these cars. The problem with them today is that few techs were ever trained for these cars and parts are almost never in stock for them now. The technical questions that arise often go unanswered at VW; they simply don't have any interest in supporting them. Dealerships are forced to find answers on their own and nearly every part that is particular to them has to ordered out of the national depot.
It's only my opinon, but some dealerships aren't keen on seeing these in their shop. They are very complex vehicles and I know from past experience the electrical system is difficult to repair. The 12 cylinder model is overkill in just about every aspect; fuel economy is poor, service access is limited and parts are extremely expensive for them. The v8 is fine motor and knowledge and parts are more commonly found, plus the fuel economy (okay, if you own one, fuel economy is the last thing you worry about) is much better than the 12, with hardly any power penalty.
Although VW dealerships are supposed to handle these for service; it rumoured that Audi dealers have been better at repairing these. The v12 motor is seen in Audi models as well, so they are more common there than at VW.
We haven't seen very many of these in the dealership; electrical problems are the most common issues; cruise control, HAV (heater/ac/vent) control system, some issues with seat heaters/massage functions, fuel pump and the like. There just aren't a lot of them around to really generalize the entire line up; haven't said that, those issues above seem to be fairly common betwen them.
While the driving "feel" is great, as you know, resale value isn't. If you're planning to buy one, think long term and decide if you can live with it until it dies. If you try to resell it, expect to take a beating on price. What the previous writer has said is true; you need to service it by the book and find someone that has patience to maintain it and live with the usual VW "service quirks." It can be a very rewarding vehicle to own and especially to drive, but it comes with a complex set of maintance requirements and expense and shouldn't be bought unless you've weighed all the pro's an cons of owning one.
I hope this helps, a car nut.
spent many of the last twenty-five years + on/off in Volkswagen dealership service departments; 30+ years in automotive service.