We just moved into a new condo that used to be a rental unit. Pretty sure they never used a filter for the furnace and it was very dirty and dusty.
Called up Clear Thumb based on their reviews. They were scheduled to arrive 2 - 5pm and arrived at 2:30 or so (calling ahead of time to give me a heads up). Charles and the crew were friendly and courteous (offering to take their shoes off for our new carpets). Very helpful too by explaining the steps and procedures.
Took them about 3 hours. Cleaned our 10 ducts, the furnace, and the bathroom vent/exhaust. The results were great. Feels so much cleaner. I'd highly recommend them for good furnace maintenance.