We recently had ours done and were tremendously pleased. I did a ton of research and felt very informed about what to look for (positive + negative). We found a service that checked all the boxes — The Duct Cleaners. If you will email me I can send you a discount code. (No, not affiliated, just was given a referral code to pass along for savings for both myself on future cleanings and for others who use it). email@example.com
I know it makes a difference. I once lived in a rental unit on Cap Hill that had a gas furnace. It was very old and neither the furnace nor the ducts were ever cleaned or serviced. I could tell when the heat was about to kick on because I would get a taste in the back of the throat. That would be the air starting to move through the vents and kicking out a little dust. I don’t know how often one should have the ducts cleaned: we had it done at the house when we got a new furnace and put in an ionic air filter and have been happy with that.