I just recently started using Reverse Osmosis water from my 3-stage RO system. My Choc Milk Stout came out great tasting as well as a honey wiesen, both extract. I did notice that both carboys had noticeable active fermenting for a good two weeks until I racked into the secondary. The honey brew I even kept it in the fermenter for an extra week and combined with the wet towel/fan chill method, fermentation was still going strong.
Does anyone have any opinions on RO water? I live in Phoenix and our water is extremely hard.