I've used an SNS in public, and people mostly don't care. The only comments I've received have been favorable, especially from older ladies who had milk supply issues and stopped nursing. They wish that something like that had been around when they had kids.
At my son's age (9 months now), I can't do the SNS in public anymore. He's too squirmy and needs quiet to focus.
It gets easier too, when the baby starts eating solids.