When they arrived, they ran a long air hose down through my duct system and covered a register with a clear box with lights in it.  They asked me to watch the box as they blasted air through the hose, and I was absolutely amazed at the amount of dust and dirt that came swirling through the box! 

I was especially surprised when a dead, dried-up mouse also came out!  I sure am glad to get that out of there!    

I have also noticed I have more air flowing through the system now.