Another option is to use detailer's clay. This will remove tar and sap as well. Once it completely clean, use a high quality wax like Klasse Alll in One on the windshield, water will bead right up while you are driving and the wipers glide across the windshield. During that huge storm yesterday, I had no problem seeing after using this treatment.
2003 530i Titanium Grey/Black, Premium and Cold Weather Package, Ultimate Pedals, iceLink iPod Kit, Bluetooth, M5 Deck Lid Spoiler, M5 Rear Sway Bar, M5 Dead Pedal, DINAN S1 Trans and Engine SW, BSW 12' subwoofer, Valentine V1 hardwired, SMG Paddleshift kit
2005 X5 Sterling Grey/Truffle Brown, Premium and Cold Weather Package, DICE iPod Kit, Bluetooth, DSP, Comfort seats, Invision DVD Headrests, Valentine V1 hardwired, TomTom GPS