well, I watched a show on the history channel the other night about bottled water. In part of it they were talking about ro/di water. They mentioned they add minerals back into the water after it is filtered, but they said it was only for taste. Apparently pure water has a "rough taste," whatever that means. If it was undrinkable I'd think they would have mentioned it.
If you've ever heard of heavy water though, that stuff can kill you if you drink a crazy amount of it. Really hard to do since heavy water only makes up a tiny percentage of regular water. Basically it's a regular water molecule, but the hydrogen atoms have a neutron on them, making it more dense, or 'heavy.'