I did it once at an old house of mine....alot of crap came out, so it seamed worth the $150 or so the crew charged
Make sure the company you hire has
That huge vacume mounted in there truck
And they drill all the little holes in the main ducts,
That they use that banger attatchment thingy,in the ducts.
And that they put those rubber plugs back into the hole they drilled, and not just duct tape