I finally signed up for this in December. Didn't notice many intercepted calls in the first few weeks but lately it seems to have "kicked in." I can tell because the phone will ring once and then stop which means NoMoRobo intercepted the call. Have definitely noticed a drop in calls from blocked or unidentified numbers in the last month. I'm not sure how they make their money but it's free to the user. Haven't seen any downside so far.
Anyone else have good or bad experiences? I put the link below for those that may not be familiar with it. I have no connection other than being a satisfied (so far) user.
http://www.nomorobo.com/