Is duct cleaning an effective thing to have done or is it a scam?

I have so much dust in my house that it makes me nuts. Two large dogs (one of which is a German Shedder) add to the problem. I change my furnace filters about every two weeks. I keep seeing ads for duct cleaning, but it seems like a total scam type of business.

If this is an effective service, how can I be sure that they really performed the service and, more importantly, will it help with dust? If it is helpful (and effective), how often should one have this service performed? Are there any reputable, national companies that perform this service?