I only know ImproveNet as a Contractor, not a customer, but my customers seem to like it, and I know I get a heck of a lot of leads (which I have to pay for) through them, but the leads are qualified with very few tire-kickers. I only suggest it because it seems to work well around here. If the customer is not being contacted by the contractors, it may just be a reflection of the economy and that just about every contractor and sub-trade I can name has been swamped for the past 5 or 6 years.
It is in my interest to promote the hand that feeds me, although I certainly don't get any direct payback for my efforts. Even if you don't get immediate results from ImproveNet, the effort didn't cost you anything but a few minutes of your time.
Still, it's good to hear your experiences, and I will keep them in mind. A quality service will survive, while a service that drops the ball on a consistent basis will not.