Originally Posted by AndreyT
Firstly, you probably got it backwards. "Almost every other car manufacturer" actually uses hydraulic steering assist.
Secondly, the actual "steering boost" in e90 cars is still provided by a traditional hydraulic system. The electric part of e90 steering would be the Active Steering system (that's probably what you were thinking about). Active Steering is an option, not present in all e90 cars. And Active Steering is not a "boost" system, it is a ratio-control system (as well as "lemme take over and steer it for you" system).
The first generation of 3-series with an electric motor providing steering boost is F30.
Steering boost and Active Steering are all components of the Hydraulic system implemented on modern vehicles including the e90. All F30's now use electric steering.