OpenDNS provides enhanced performance, eliminates some problems, improves control and security, and of course, it's free. Granted, it doesn't do everything. You will still need internal DNS for systems on your local network, behind NAT, or in a VPN. Since OpenDNS does not intend to provide those features, I can't suggest that it's a weakness. Overall there are plenty of reasons to use OpenDNS, and it's easy to give it a try. Highly recommended!
To learn more about using OpenDNS, be sure to read our article Use OpenDNS To Improve Your Security,