[Skyhigh]

The basic principle is that xDrive is a full-time AWD system with dynamic torque-distribution default set to 60% rear, 40% front.

There are two (main) exceptions from the above:
1. Low speed manoeuvring uses primarily or even exclusively RWD drive
2. High speed (>180 km/h) also uses 100% RWD

The two extreme points of the torque distribution are front:back 50:50 to 0:100, meaning the maximum torque which can be mechanically sent to the front is 50%, minimum 0.

There is another thread where the term "virtually" has been beaten to death (some sources claim 100% torque can be "virtually" applied to either axle). This is "virtually" true only in the case that both rear wheels lose traction entirely, in which case clearly 100% of the momentarily available torque is used by the wheels with traction - in this case the front. The key here is to understand the secret meaning of "virtually", as realistically the front wheels can only get a max of 50% of the drivetrain torque. This is a straight-forward mechanical limitation (clutch fully engaged).

To conclude - xDrive is totally worth it for:
- Winter (snow/ice) driving
- Fast acceleration from stand or low speeds
- Overall stability during sporty driving (no corner too sharp)
- Overall better drive performance

Downsides:
- Added weight
- Increased drive-train losses
- Increased fuel consumption (due to the two above)
- Slower acceleration when sufficient traction is there (e.g. 100-200 acceleration suffers in comparison to RWD)
- Increased maintenance burden

I would not have bought a BMW without xDrive and I do not regret that decision.

Also keep in mind - you can always disable xDrive if you really want to (e.g. by using tools like xDelete), but you can never add xDrive.

Hope this helps.