I use tap water, fresh from the tap
(1) it's easy
(2) it's relatively cheap
(3) it's hard enough for brewing but not too hard
(4) we don't have a ton of chlorine in the water
I don't boil it because it seems to work out fine without boiling. Never had a problem. But tap water is pretty good IMHO
A big reason I brew is economics. I like decent beer, but I don't like paying out $10 for 72 oz when I can pay $30 for 640 oz of like-quality beer. Cost is a factor. Sure, I could buy water and control the mineral content manually, and perhaps get a better beer, but how much better?
Maybe I'll try brewing with store-bought RO water and add minerals sometime and see if it works out better, but I need to fine-tune fermentation temps and pitching first (that's job-one right now).