My experience with an '08 535 taught me that BMW is actually German for "check engine light." Which leads to an inconvenient trip to the dealership (or as many as three trips in a 27 day period) that results in yet another reset of the software. You can get BMW NA involved and speak to a very considerate and empathetic customer care rep. Sometimes they can even convince the dealer to actually look under the hood where they will find spark plugs saturated with fuel, misbehaving fuel injectors, in addition to a bad HPFP ... Never have I had a worse ownership experience.
But, here I am, considering another BMW, even with all the grief, aggravation and of loss of good will. My wife wants an X35d, so I'm checking on the forums so we approach this purchase with our eyes open.
All that said, my colleague has not had a single problem with his '11 X35d in 10k miles (although had a hell of a time with his '04 530i). My sister-in-law has not had a minute's issue with her '08 335 in 110k miles. Many customers have very happy ownership experiences with their BMWs.
If you want assurance of a dependable, reliable car, a BMW (or any German make) is probably not your best choice. Japanese and (horrors!) American badges should be higher on your consideration list. My company leases me a new Ford every two or three years for business use. Every one has been dull, uninspiring, boring and been rock solid dependable and reliable.
You buy a BMW for that little tingle you get at the base of your brain stem when you nail the accelerator or take that S-curve faster than you really should. Just don't expect a trouble-free ownership experience. If you are lucky enough to have one, Great! You will be even more hooked. If that BMW (German, meaning "check engine light") comes on, you won't be surprised. Even after the 3rd or 4th time in a month.
When it comes to the diesel version, keep in mind that these are the engines that the engineers and designers in Munich are driving in their daily lives. Although not common or popular in the USA, 70% of the passenger fleet in Europe are diesels. These engines probably have more fleet miles on them than BMW's gasoline variants have.
These are my experiences and opinions. My best wishes for safe and happy driving.