Okay Mike this is totally MY opinion..........
It is a waste of money if your house is newer (10-15 years).
I know the backlash is coming but i will speak my peace. I know a little bit about HVAC:
and if you think this through it will make sense.
Where does the air that goes into the ductwork come from? Your furnace right? In order for that air to get from the blower wheel to your duct work it must first pass through your filters right? of course. NOW-- The dirtiest part of your ductwork is obviously your registers AND the 2 feet back or so.
I recommend you remove the register covers and take a flexible hose from your wet vac and push it into the opening and vacauum those areas real well. You would be surprised how dirty those are from stuff falling INTO the register.
To prove my point-- you can go buy a sheet of filter screen and place it under the register discharge and see how very little dust there is coming out.
HERE is when you WANT your ducts clean.....
1. When the bozo finishing/sanding your hardwood floors forgot to turn off your furnace
2. New walls put up and when sanding they forgot to shut off the furnace as well.
Just my opinion mike so take it for what its worth.........