I've used the RotoBrush, it's crap. Unless you want to clean the entire house after you're done, don't fool with it. Oh it will break the stuff loose in the ductwork, but it won't suck it up. So just imagine the mess you have when you turn the system back on.
Rotobrush is a cheaper alternative to the real machines (i.e. Abatement Technologies) which will cost you about $10k for a full set up with compressor. The rotobrush product is targeted to carpet cleaners and hacks who dont want to invest in the equipment and trucks to do the job right. They give ductcleaning a bad name!