I gotta say...
I've dealt with Tesco on the supply side of things and they are on the whole pretty decent and not too hard on pricings with suppliers, contrary to public conception of them...
Now Dunnes on the other hand, are the opposite... I wondered was it just personality clash, then I read that Irish Times stopped supplying them with their paper, followed by the Guardian. Both papers took the unprecedented step of taking out full page ads in their papers explaining why they had stopped the supply and listed (in some detail) the alternative outlets adjacent to each Dunnes that would supply Times and Guardian.
Hopefully that helps inform any micros thinking of taking the leap..