It's by no means a constant, permeating stink. Sure, there are some neighborhoods with open air food markets that can be smelly, or some that are downwind of waste processing centers (most of East Williamsburg), or smelly because of wastewater (parts of East New York and Canarsie), but for 95% of the city, you're not going to smell anything unpleasant unless you're leaning in to sniff trash bags.