Duct cleaning results vary depending on how dirty the system actually was to begin with. Managed a restaurant space for three years and ventilation became noticeably worse over time. Brought in ventilation duct cleaning services after customers started mentioning stuffiness. The difference was pretty obvious once they finished - airflow improved and the kitchen didn't feel as hot during service. Heating costs did drop slightly the next month though hard to say if that was entirely from cleaning. Worth doing if you haven't had it done in years and notice air quality issues