My biggest problem with my neighbor's chickens was that they kept getting out an coming over to my property and he'd have to ask me to let him in the back yard so he could take them back home. Never noticed any smells near their coop which was near the property line.

Never really bothered me since they mostly just hung out around the back fence far from my house, but finally another neighbor complained to the city and they had to get rid of the chickens. Chickens are allowed here (up to 10 per property), but have to be kept confined and on your own property.