Maki Enterprise is a Japanese Company. There is no real disadvantage to this though, just the entertaining poorly translated documentation. The interfaces of their products are decent. KidServer is highly customizable to do much more than just ordinary filtering and can be administered from any browser. Kids GoGoGo can also set time restrictions on internet access and restrict access to specific applications (which you can also do natively on OS X). Kids GoGoGo is $30, and KidServer PE is $40, compared to $59 for ContentBarrier. KidServer costs $180 for a 1 year 5 user license.
Maki also offers a daily updated database for Kids GoGoGo for $24 per year. Unlike the default block lists, the Daily DB is hand harvested by Maki employees. I don't know how Intego's update software works.
I would highly recommend that you download Trial versions before buying. Both Maki and Intego offer free evaluation downloads from their websites.
ContentBarrier/NetBarrier seems the most polished of the products, but Kids GoGoGo, KidServer, and KidServer PE are also very good products. KidServer has the added advantage of being highly scalable. As far as price is concerned though , Maki seems the way to go.