Everyone is trying to extrapolate the long term reliability. But the fact is we have no solid proof yet. My take based on the fact BMW has been building Diesels for decades is the base mechanics of the engine is very reliable.
I think one area of concern is the same for any modern high pressure direct injection engine, fuel delivery system and injectors, including gasoline engines.
The one potential issue for the D is around all the emission controls and what appears is a tendency for excessive carbon build-up in the EGR/Intake system. If you are willing to accept you 'may" need to have this cleaned one or twice in 200K miles, then very likely you will not be disspointed with the car.
I would expect that if this indeed becomes common, more efficient procedures will be developed and the cost of the cleaning will come down. I still have yet to determine if the head really needs to come off to clean the intake paths, or can everything just be vacuumed out when done in place.
I am currently at 28K (I know not much), have never used any additives (just like BMW recommends). I've not seen an SES yet. As my car was a service loaner for the first part of it's life, it was likely driven hard before all the EGR updates were done. This probably was a good thing! The only special thing I do is to only purchase diesel from stations that seem to have a lot of activity, so the fuel will be fresh. I don't go to that station down the road with ONE diesel pump, that I NEVER see anyone using.