With plumbed-in espresso machines, one other problem with straight RO water (or distilled) is that it is relatively mineral-hungry. Over time, it will leach minerals out of the piping and boiler, which isn't a good thing. Though it may never be a problem with the quantities of water in home use.
The best RO-based solution I've seen for coffee use is the "Easy" from a company called Cirqua (http://cirqua.com/
). They work by taking RO water and adding a controlled amount of minerals back into it to get it to the ideal level of dissolved solids for coffee. It's not exactly a practical solution for home use, though.
A simpler, lower-cost option is the Claris line from Everpure (http://www.everpure.com/newspress/pages/claris.aspx
). It's a similar concept, but instead of RO water with minerals, it blends softened and filtered water together to get closer to the right concentration. While softened water is not ideal for many things, it can really improve the flavor of coffee. I have a plumbed-in espresso machine, which is supplied by a small softener-only cartridge (had I known about the Claris when I set things up, I would have gone that route). My city water is not overly hard, but it's not soft either. The flavor difference between espresso shots made before and after I installed the softener was really amazing, even though the water is actually softer than the ideal. Once the current softener cartridge is past its prime, I plan on replacing it with one of the Claris units, unless something better has come along by then.
And if you haven't already, I highly recommend reading, or at least skimming, Jim Schulman's "Insanely Long Water FAQ" (http://www.big-rick.com/coffee/waterfaq.html
). There's a lot of great information in there about the mineral content of water, how to treat it, and particularly how it all pertains to espresso.