I used it for a while early on but eventually got fed up with occasional false positives where they limited access to a legitimate site (which in each case, after switching to my ISP's DNS, were fine). I switched to Google's DNS and never had a problem after that.

I believe they've added customizable filtering, etc, in the interim, but the idea of a DNS provider restricting access to anything just didn't sit right with me in the end because of the risk of false positives (systems like SURBL have a similar problem).