Regular maintenance is must as your aware. You may not be aware that BMW says the transmission fluid is a "lifetime fluid" and does not need changing. However most people disagree. As with any fluid it will break down over time. If you notice issues with the transmission the simplest fix may be simply replacing the fluid.
If you take good care of them they will last a long time. Im not to firmilar with the diesel models so i cant speak towards what problems they may have.