RO (Reverse Osmosis) water is not ideal to use for brewing by itself, but is great to use for "water profiling". By starting with a "basic" water that is void of brewing salts, it allows the brewer great flexibility to add specific salts back as needed to reach the desired profile. I use RO water at a rate of 2/3 added with my regular water (moderately hard) to create suitable water for European lagers, etc...
Note: Be certain the RO filter you use for brewing has a carbon pre-filter to remove organic chemicals & chlorination.
I am super new to brewing and ordered a Mr. Beer "dump in and stir" kind of kit and used RO for my first brew. I am also into salt water aquariums so I have an RO at the house and figured it couldnt hurt.
RO water is stripped of allmost all minerals, It gets everything. I have a home RO system and use it for a few brews. I went back to using just my normal softened water for the most beers.