I think I read once that the water softening filters (like the brita 4 stage, I think its called ion displacement) is bad stuff for brewing. It replaces calcium salts with a potassium salt or something or other which doesnt do wonders... (apparently... haven't tested it personally )
Any ion-exchange type "softener" will remove calcium which is not what you want. This includes the "Brita" filters.
The best results are using RO water (closest to distilled) and then adding back salts (calcium + magnesium salts) into the water. Other than that a plain old carbon cartridge that will remove chlorine or chloramine and most contaminants will do enough.