I have a call blocker installed at home, and it is awesome. When someone calls from a blocked number or a "Private" or "No Info" number, my house phones never even ring.
The only problem is that it commonly blocks known good callers if the phone company doesn't display the caller ID info fast enough. It keeps me from calling the house from my cell on a fairly regular basis. No bid deal, just redial. I have had complaints from several family members that it keeps dropping their calls, but that is a small price to pay to be able to watch an entire movie without someone interrupting me to try and sell me something via cold call.