Alexandra, my husband is very sensitive to any kind of second hand smoke. We haven't had problems in cabins, but have had issues a couple of times on our balcony. On our recent cruise to Alaska, one man chose to smoke a cigar on his balcony. His cabin was forward and down a couple decks from us. I guess he didn't read the Princess smoking policy which states smoking cigars outside of Players Bar lounge is not permitted.
Generally speaking, though, we haven't had any big issues regarding smoking on any of our cruises.