I grew up on hard well water softened with a water softener, so after moving into an area where there is municipal water supply, I noticed the difference immediately. I still cannot stand the taste of the municipal water. I don't drink if I'm thirsty.
I run my water through a 1 gallon Brita filter (thats all I have at the moment). The taste improves about 85%. I think if the water tastes good, it is good for brewing in terms of off favors.
If you are going for a clone and you want the water to be that of something like Czech or Germany or Belgium, then treating the water to your needs would be how you would want to do it.
As far as water, if you can make it taste good, I think it is fine for brewing.
If you have areas of spring water from a mountainside or something that is constantly flowing, I have heard of it adding a unique falvor to beer as well. I have no problem finding that here in Alaska, but in Tenessee, not sure