Duct cleaning is heavily marketed, and whether it's worth it depends on your situation. It's genuinely useful in some cases and unnecessary in many others, so it helps to know the difference.
When it helps
Duct cleaning is worthwhile after a renovation that put drywall dust through the system, if you see visible mould in the ducts, if there's a rodent or insect infestation in the ductwork, or if ducts are clearly clogged with debris. In these cases it improves air and airflow.
When it's oversold
Routine duct cleaning of an otherwise clean system shows little proven benefit for air quality, and the very cheap deals are often bait for upselling. A clean furnace filter and regular HVAC maintenance do more for air quality than periodic duct cleaning in a normal home.
Choosing a service
If you do hire it, choose a reputable company that uses proper equipment (powerful vacuum plus agitation), cleans the whole system, and gives a clear price — not a rock-bottom 'special' that becomes a high-pressure sales visit.