I'm on beautiful, wonderful Colorado well water. Love it, but the same rules apply to all water supplies. Well water just happens to be extremely variable, because you might have very different water than that of your neighbor. Here are my three rules:
1. Test your water.
http://www.wardlab.com/ will do you the right test for $16. You MUST know what you're working with to create the best beer you can.
2. Filter for chlorine, if that's part of your system. This, to me, is a non-negotiable if you've got noticeable quantities of chlorine in your water. Phenols are nasty, except in Belgians.
3.Adjust your water for the style of brew you're creating. The best resource for this is Palmer's online book (
http://howtobrew.com/section3/chapter15.html). Remember that water not only effects the mash ph, but also the finished character of the beer. Depending on your water supply, you can do everything else right and your finished beer may not just taste right. The Brewing Network recently did a four-part series on water that addresses many of these concerns. Palmer goes through his spreadsheet and it's very informative.
http://thebrewingnetwork.com/shows/Brew-Strong/Page-2
Overall, you might have great water for one style, but not for another. Remember that quality in = quality out. If your water is crap, you're never going to make great beer. Better off buying RO water.