This article is from the Water for coffee FAQ, by Jim Schulman with numerous contributions by others.
I regret that I could not find enough information, nor had the
testing resources, to let me recommend or demur the many available
brands of water treatment. Given what I've learnt, I find the selling
tactics of almost all that I've seen completely reprehensible, as they
trade heavily on fear and ignorance (yes Virginia, you will die if you
drink tap water). However, it could well be that some or all work as
they should. I'd welcome reports by users of these units.