Starter failed -- very hard to replace yourself.
Front ball joints, thrush bushings -- not too hard to replace yourself, but properly tensioning the thrust bushings is a pain unless you have 450 lbs. of ballast and an old-fashioned grease pit.
Front brake pads and disks -- easy to fix.
Rear brake pads and disks -- also easy.
Leaky radiator -- surprisingly easy to fix compared to some cars. It is made of aluminum and plastic, so it is amazingly light.
Viscous fan clutch -- done at the same time as the radiator... very easy.
SRS (airbag) warning light -- diagnosed by a dealer as a $1200 replacement of the main SRS controller. Since the system is disabled when the light is on, I've just decided to skip this repair.
Hydraulic reservoir level sensor -- integral to the hydraulic fluid tank, so I'll replace the tank next time I need to change the fluid.
Leaking fuel hoses -- extreme engine compartment heat just cooks these little rubber fuel hoses which connect metal fuel pipes to the fuel rails. Replace them with fuel injection hose instead of regular fuel hose.
Fuel pressure regulator vacuum hoses -- also cooked by the extreme underhood heat... easy replacement with simple fuel hose.
Drive shaft -- the center bushing was used up and the U-joints were seized. Replacing this requires dropping the very heavy exhaust system from the headers back.
Oxygen sensors -- these needed replacing after 50,000 miles, so I did them when I had the exhaust system dropped when replacing the drive shaft.
Rear head rest -- when I strapped in one of our children's car seat, it trapped the rear head rest in the down position, so when it tried to extend automatically it stripped its nylon gears. I haven't repaired this yet.
Battery -- normal replacement item, but when it was failing the slight low voltage conditions resulted in many puzzling errors and alarm system false alarms.
EML errors -- in one particular town while driving down I-5 in Oregon, the car has suffered from EML errors on six occasions, which shuts down one of the two banks of cylinders and effectively puts the car in "limp home" mode. (Simply shutting the car off for 10 minutes allows the Motronic computers to reset, and you can drive away.) We realized belatedly that this was caused by electromagnetic pulses from a local heavy metal smelter.
Routine maintenance -- some of the 12 spark plugs are very hard to get at... very hard. Everything else is just like any BMW 6-cylinder, only two-fold. Two distributor caps and rotors, two air filters, two fuel filters, etc. Otherwise, this is just about like any other car to maintain.
This is definitely the nicest car I've ever owned. Although it requires almost constant attention, it has only twice required a tow: once because of a failed starter (the planetary gears inside it were smashed), and again when a radiator hose popped off the radiator on the freeway (my error... I didn't tighten a hose clamp enough after changing radiator hoses).
The car is very reliable on the road. I especially appreciate an engine that can run on six cylinders if an error occurs that would cause most other engine management systems to shut down most other engines entirely. Parts costs can be high, and dealer labor costs are ludicrous. There is no way you can talk yourself into believing that this car is a bargain to own or maintain... it is only manageable if you do your own work.
On the other hand, in the realm of V-12 luxury autobahn titans, this is about as affordable as you're going to get. What a car! It is effortlessly fast... so smooth that it feels like it is powered by an electric motor or a gas turbine. This V-12 has an absolutely unique sound at full throttle, and any passenger that hears it always remarks about it. The handling is about as good as it gets for a 4500 lb. luxury car. It covers ground very quickly, especially the twisting roads of western Marin, rural highways of Oregon, and mountain roads anywhere.
Even after all these years, the car still has cachet. We'll occasionally come out to find that valet parking attendants have parked the car right up front where rich people expect to find their car. Of course the car was designed to be a limousine in Germany, but for us it is a very nice, comfortable, roomy family sedan with good handling and power.