This is definately one of the best BMW's I have ever owned! I have the Auto and it really does give a beautiful drive and very responsive! When you put your foot down it goes!
When I got the car it still had the catalytic converters in it, and had quite a bit of turbo lag, I removed the cat and put a 63mm down pipe and its a whole new machine! All the little sounds it makes from the turbo are now audible and sounds awsome, The consumption also went right down! All I have to do now is software! I also did the EGR delete mod, which further improved consumption, and much less smoke gets pushed out the back when I put my foot down now!
I always hated Diesels and thought because of the low RPM that they would be a terrible drive, But with the Steptronic box the drive is incredibly smooth, the shifts are almost seemless, if you turn traction conctrol completely off and put the car in manual mode its a frikken monster! and this is with stock software! I can only imagine what this car feels like when its pushing over 500nm of torque on the wheels!!
Now I know there is allot of negativity towards this cars autobox! but from what I've read there are plenty mixed reviews on it, some people out there have mapped their auto's to push over 700nm and drive the car for years with no problems, and very few have stated that their boxes have actually broken but most have said that the box give shit once you change the Automatic transmission Fluid which is rated by BMW as a life time fluid so should never need to be changed.
I also think that its down to how you drive the car, if you drive it like a monkey everyday and tramp the accelerator in 1st in manual mode when the car is mapped which puts immense amounts of torque on the box then I'm sure it will break sooner than later too! a good Tuner should be able to map the car in such a way that the bigger amounts of torque come in 3rd gear and above instead of 700nm+ in 1st, which will not only be dangerous for your box but for drive shafts etc etc too!
Anyway hope this helps, just thought I would give my 2c on the subject!