I am also experiencing this same issue of brake squeal at low speeds. On the E90 forum here I have seen multiple threads with this issue. I think BMW needs to man up and do a recall on these brakes and fix the issue for once and for all.
As there are way too many 335i's that are suffering from this design and material flaw of the OEM brakes.
Maybe BMW will only step up and do that if a mass law suit is filed much like in case of turbos waste gate rattle. Come on BMW fix this issue a over 50K luxury car should not squeal like a pig. It is embarrassing when a BMW sounds like a dump truck while braking.