This is fantastically impossible.
It will about as stable as any other commodity plus the additional swings due to public stampedes one way or another, and the fact that there is no 'underlying value' to anchor the price (like Oil).
Feasibly, if exchange rates are razor thin (unlikely) then it might be possible buy milk and bread with BTC simply by doing the conversion on point, but that of course requires someone to do some 'investing math / intuition' every time they buy a stick of gum.
It can't be a functional currency, at least as it's designed.
And the notion that it consumes gigantic amounts of energy for no net value gained should have everyone up in arms.