I currently have an E46 330d Auto and do 25k a year. It does 38mpg but even though this is well behind a 320d its still far better than other stuff I considered, e.g. subaru imprezza 20mpg etc.
As my mileage demands are going to drop soon, I was thinking of getting a new 320d 177, principally as its quick and the economy is very impressive. After a test drive I came away thinking it was as quick as my 330d in practice - great. I then test drove a E91 330d and realised life was too short to worry too much about those last few mpgs between a 330d and a 320d - do the sums, compare the savings from:
a) 20-25mpg, (petrol monster)
b) 38mpg and (330d)
c) 50mpg. (320d)
The big leap is from a) to b) - the increase b) to c) is much reduced and put against how much bigger the smile on your face from a 330d will be, not worth it.
All IMO, but for these reasons its now 330d/335d E91 only for me. Great car though it is the 320d is off my short list!