I have an F10 530d and, after my E60 530d, the engine provides more than sufficient power. I don't know how or when I would use the extra power of the 535d, but of course, if you can afford the extra power, why not? I bought my car off the floor with some very nice extras which I would not want to do without. Here in South Africa the 535d is FAR more expensive than both the 530d and the 535i. Some of you may find the figures interesting (bearing in mind that the South African Rand is (today) trading at R6.90 to the US dollar:
523i: R484 728
520d: R486 453
528i: R574 228
530d: R633 420
535i: R652 412
535d: R737 591
550i: R845 516.
While the prices are high, the standard specification is quite good. For example, 530d and up come standard with the Sports Auto, but not adaptive drive.
I don't think that the difference in price, at least in South Africa, would be worth it for an extra 40kw and 60nm. One would possibly also have to bear in mind the manner in which a diesel develops its power. I think one would be hard-pressed to tell the difference between 540nm and 600nm of torque. And I hardly ever have the opportunity or a long enough stretch of road to rev my car to 4000 rpm, the point at which maximum power is developed. While the 535d has two turbos, I don't think that the 530d can be accused of lethargy. In fact, Fifth Gear expressed the view that the 530d was marginally more responsive than the 535i. (
http://fwd.five.tv/cars/executive/bmw-5-series-2).
Mikestrete