Share 'Duct Cleaning? Good value or a waste of money?'

I have lots of my auditing customers who ask me about duct cleaning, and if they should do it.  It seems like the only way to get them really clean is with some sort of roto brush, but it also seems like that process is going to possibly do more damage than good.  Obviously this really depends on the duct system and the quality of the craftsmanship, but I was just looking for the Pro's opinions.…