You could use it in a pinch. I don't know enough about it to say for sure what long term use would do. We sell demineralized water here also, but it's right next to the distilled water.
I have a distiller that makes about 1.5 gallons or approximately 6 liters (after boiling for 6+ hours). Too back up previous claims, yes it does use a boat load of energy. So in emergency situations, I'll boil up a batch. Yes, I've gone to walmart and seen no distilled available and desperately needed it.
For reference: $0.89 per gallon (about 4 liters) for me at my local store. Energy costs to make it on my own are roughly 8 times that much, not to mention the expense of the distiller (they are not cheap).