Here's my experience: I have owned my 1990 Bentley Turbo R for less than a year so I don't have a long history. I own two other cars (2006 Audi A4 and 2004 Porsche Boxster) and I put about equal miles, maybe 5000 miles/year, on each. So although it's not a "daily driver" I bought my Bentley to drive it. There's a small, private Rolls-Royce repair shop not far from where I live that's been there since I was a kid. That's one of the reasons I felt comfortable buying the car - I knew I had a reliable place to bring it. I expect to have the Bentley in the shop once or twice a year for routine maintenance and to fix the stuff that is going to fail on a 20-year-old car. For example, two of the four power window mechanisms have failed since I've owned the car. Repair costs do not seem inordinately high for this car as compared to other cars I've owned, nonetheless I'm expecting to average about $5000/year in maintenance and repair.
One thing I'm not expecting is power train problems. This vintage of Rolls-Royce/Bentley has a pretty solid reputation for reliable power trains. I worry more about failures of auxiliaries - fuel pump, alternator, etc. - leaving me stranded, though that hasn't happened yet.
For me, the real key to owning and driving and enjoying a 20-year-old car - Rolls-Royce or otherwise - is to have at least one other car, preferably one that's more modern that you can beat on. An A4 is a good beater. My Bentley doesn't have to see bad weather, doesn't have to go to nasty places (I live in New Jersey) and can stay in the garage if she's not feeling up to par.