I have the standard BMW ones fitted - seem to protect hands and controls well enough. However, I also fitted the Touratech extra guards that screw into the TOP of the BMW guards - I think they give a little more protection from rain.
I think that fitting guards of any kind will reduce the wind chill effect in the cold PLUS offer more protection in the rain but sooner or later your hands will get cold / wet - they are certainly NOT a miracle solution to wind / rain! Coupled with heated grips, however, your hands will stay warmer for longer in the winter, IMHO.
A decent investment, that I would certainly miss on another bike.
Nick