I tried once when DD was very young, but it was such a PITA that I never did again. I needed extra hands to manage baby, boob, clothing and cover. Since the cover was the only optional part of that equation, it got ditched. I've only had one negative comment and I nursed everywhere all over the Southeast. (generally considered a pretty conservative area)
My personal theory is that the moms who NIP w/o covers are so matter of fact about it combines with the fact that almost no one expects to see a woman nursing (gasp) with out a cover (shock and horror) that the average person never sees the uncovered NIP-ing that's taking place right under their noses. Therefore people think covers are necessary, because surly NIP w/o one would cause a stir. They only see covered mamas nursing since a cover is a big sign shouting "NURSING HAPPENING HERE"