Leave it on, leave it "off"; for 90% of conditions and 100% of dry pavement, you'll not notice the diff.

But, I'd leave it on...

The only time it helps to punch it off, is climbing Very steep inclines in slippery conditions, eg snow/ice, loose gravel:
in those rare and brief conditions, punching off the DSC allows
you to add more throttle or even floor it, and prevents that
nasty, sometimes wrong, "traction"/brake grab action to come on.

Ya'll make mtns out of molehills.

so its not a real time 4x4 its AWD only?

i think it only says 4x4 on the newer X's

YES!

In the model years Before XDrive, it was AWD, with a
Constant 40-60 split from frt to rear. AWD, all the time
and that ratio never changed/changes. The DSC is a
smarty pants 'puter that senses pitch/yaw, wheel position,
tire rotation at each wheel, etc. It "corrects", as best it
can, via the braking system at each wheel, situs that seem to be "dangerous" to the smarty pants computer. It works almost all of the time, seamlessly and very well. The only time I disengage DSC, as reported above, is to allow Me the Driver, to gas it and spin the damn tars, and prevent any individual wheel braking by the system.

On newer MYs, the XDrive can send Up To 50% of the Torque to the front wheels in an instant. Torque is the key word, here. Its DSC works nearly identically, but power and Torque, can be Adjusted back to front,
unlike the pre-XDrive models.

Is it 4 WD? Not really, imo. But, it is AWD, and the newer
XDrive is titz.

This is a layman's definitions from an ol'guy...the engineers
and techheads here can explain more eloquently, but that's
it in a PeanutGallery nutshell, pun intended.
GL,md

Here's how the BMW techheads explain DSC
Dynamic Stability Control (DSC) not only optimises driving stability when starting or accelerating, it also improves traction. In addition, the system recognises unstable driving conditions such as under - or oversteering and helps to maintain a safe course for the vehicle.
Advanced sensory technology monitors the wheel rotation speed, lateral acceleration and the yaw rate (rotation around the vertical axis). This data is processed to obtain information about the automobile as it is currently moving, and is compared with data about how it should be currently moving, as supplied by the accelerator and the angle of the steering wheel.
If these two lists of data do not match and the automobile's stability is threatened, DSC is activated and influences the brake system or the engine management. This action helps prevent dangerous situations such as skidding.

Here's an explanation of xDrive working with DSC

Dynamic Traction Control (DTC).
Compared to Dynamic Stability Control (DSC), Dynamic Traction Control (DTC) provides rear wheel vehicles with greater propulsion. This increased strength serves different purposes depending on the type of automobile. In the 5 Series and 7 Series, for example, it is ideal for straight stretches of road, and for maximum power when driving in snow or sleet.
In the 3 Series and Z4 Roadster, DTC also plays a role when on bends - it enhances the car's propulsion and its sports-style qualities. The driver is in full control of their vehicle within a predetermined parameter: the regulatory functions are inactive. DSC optimises directional stability within certain boundaries.

Personally I notice the DSC all the time. During hard half circle traffic turns or tight hairpin turns it cut the motor and it feels bogged down. I leave it on for everyday driving (since I forget to turn it off everytime) but rather have it off for me and on for my wife taking it for a drive occasionally. With it on it doesnt allow for any wheel skid at all. Not for getting the most out of the car.

What makes a 4x4 or AWD then? My previous car, a "real" 4x4 was a landcruiser. Now, it had 3 diffs, the centre one you could lock up so the front and rear axle would be powered together. The powered result is what X drive does, it can proportion 0-50% of drive to the front axles. The rear is always driven, as you all know I'm sure. So when the clutch pack is fully engaged, its the same drive as a "real" 4x4. Up to that point though, its proportional.

So I think the AWD thingy was started by subaru, terminology, not a description of the mechanical details.

Anyways, we all drive a SAV, so there!

Mat
3.0d