Since I live on top of one of the nations finest aquifers it seems a shame not to use it as it comes out of my tap pretty much free with just as bit of chlorine in it. I mix up five gallons with a crushed campden tablet before brewing, some for the partial boil and the rest in the fermenter. Like they say, if your tap water tastes good, I say use it.
What I would like to do one day is make a beer with water directly from one of the local springs but I haven't figured out how I would collect it (a full boil of course!).