This Bon-Ami FAQ and reference to BMW bulletin is old and I'm not sure if recent formula changes affects the cleaning ability but some bimmer folks swore by it for cleaning windshields.
http://www.garageboy.com/bmw/bonami.html
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2013 x35i (Space Gray, MSport, CW/CWII, DHP, Premium, Premium Sound, Technology, BMW-Apps)
Retired: '09 C300 4Matic, '09 GTI, '06 750i, '05 C55, '04 R32, '03 M5, '02 GTI, '01 M5, '00 M-Coupe, '00 540i/6, '99 M3 Coupe, '98 M3 Coupe, '96 328i,... '86 4000 CS Quattro... too many to list.