I was doing 100 miles a day at one point in my 530 during the first year I had it.
We actually drove both the 530d and the 530i back from the Midlands on Sunday in convoy, the 530i returned 36.7mpg and the 530d 42.9mpg. Over 200 miles, this works out as:
£21.80 in the diesel
£23.25 in the petrol
Epic diesel savings there. Then one day the turbo will explode and cost a grand to fix.
Don't get me wrong, I like the 530d. It is a brilliant car, the engine is fantastic, the power delivery is good and its reasonably economical. But it was CONSIDERABLY more expensive than the equivilent 530i, it is a considerably more complex engine which can only become more of an issue as it gets older, it isn't as fun to drive as the 530i, it isn't as quick and it isnt really any cheaper to run.
Frankly if they are so close when we are looking at one of the best diesel engines ever made (I beleive it was engine of the year throughout most of its production life and it's the same basic block thats still in use today) then I can't see it being anything other than worse for the diesel when we look at some of the more nasty, noisy and agricultural engines such as the 4 bangers found in Golfs etc.