When I test drove a 520d about a year ago i decided not to buy the F10 because of the constant small corrections required when motorway driving - it was terrible. Checking with a couple of my colleagues who had F10s, they confirmed that their cars had this issue although it didn't seem to worry them so much. On A and B roads the drive was great.
A year later I decided to give the F10 another chance and test drove a 525d M Sport and a 530d M Sport both with 19" tyres and this problem had gone away. Didn't want the M Sport so went for the 530d SE with Adaptive Drive and 18" tyres (all the reports indicate that the 520 is set up around 18" tyres) to make sure I got the best possible handling.
Although I might have gone for the Adaptive Drive anyway, it is disappointing that I thought I had to spend an extra £3k just to ensure I got a BMW that drove like a BMW.
I'll tell you if I go it right in about 4 weeks