A small number of products are currently registered by EPA specifically for use on the inside of bare sheet metal air ducts. A number of products are also registered for use as sanitizers on hard surfaces, which could include the interior of bare sheet metal ducts. While many such products may be used legally inside of unlined ducts if all label directions are followed, some of the directions on the label may be inappropriate for use in ducts. For example, if the directions indicate "rinse with water", the added moisture could stimulate mold growth.
We bought the Groupon deal. One of the techs knocked a pitcher off a shelf in our kitchen during the service and did not tell us about it, he just picked it (and the two broken pieces that came out of it) up and put them back on a different shelf. We didn't discover that it was broken until later that evening. We managed to speak with one of the two guys that evening, who said it wasn't him, must have been the other guy, (and still had the nerve to try to talk us into an additional $250 worth of service with them!) and said he would talk to his boss and have someone call us. That didn't happen. I have called the office several times myself in the two weeks since trying to speak with a manager, and my husband has also called. No one has bothered to call us back yet to address this or offer any sort of recompense for the broken item. Therefore I can't recommend this company to anyone else.
You may consider having your air ducts cleaned simply because it seems logical that air ducts will get dirty over time and should be occasionally cleaned. Provided that the cleaning is done properly, no evidence suggests that such cleaning would be detrimental. EPA does not recommend that the air ducts be cleaned routinely, but only as needed. EPA does, however, recommend that if you have a fuel burning furnace, stove or fireplace, they be inspected for proper functioning and serviced before each heating season to protect against carbon monoxide poisoning.