I've read/watched reports on these and as good as they are, much are 'too good'.They will block too much out.
IE: a search for info on breast cancer was blocked. These are the types of problems that were experienced. Not sure what you unltimately want to block out - but some of these adult site proprietors are slick as well. They'll insure that they come out on the mildest search.
Apple's earlier system was the best for allowing children, etc access to the internet without the problems of most filtering systems. Instead of actually blocking content based on certain criteria, it allowed users to only access a massive list of pre-approved sites suitable for children. The site rankings were done by a team of educators.
Unfortunately, due to lack of interest, the program was cancelled (it worked very well, and had hundreds of thousands of approved sites, but users were lukewarm or probably simply didn't know about it).
Content Barrier works OK (when Apple cancelled their service, everyone who was enrolled got a free copy). It's really your only choice now, so no matter it's flaws (the same as any blocking filter), the point is kind of moot. Use it or nothing.
You can also set up a UNIX utility called SNORT which is a Network Intrusion Detection System (NIDS) that runs in the background. It's essentially seamless. SNORT is designed for use by individuals and corporations, and thus has a Porn (and P2P, etc) alert system to keep employees from playing around or doing unauthorized stuff at work on company computers. The disadvantage is that it's simply an alert system; it doesn't block content. It will, however, alert you to the behaviour, either via a desktop alert, a system log, or by emailing a given address for each alert.
The OSX version is called HenWen. Read the manual and go through the step-by-step configuration instructions (you can use the defaults, but the porn filter won't be on, for example). Because it is very robust & flexible, you have to configure it for your individual situation. It's free.
Your best solution is to purchase a firewalling system with a "subscribable" list that the firewall downloads off of a content filtering server each day/week/month and is constantly updating. Even with these, there are ways to get around them all.
Another option is to use a Mac OS X server as the router/firewall and set up the proxy server to do your content filtering. You can add and customize this list as your needs grow to block more and more sites.
While this is probably one of the cheaper routes, it requres more work on your part. Plus with this being a "seperate" box on your network, kids will have less chance at disabling the program from the desktop.