Google DNS vs OpenDNS

On 3 December 2009, Google launched its Public DNS service. Google Public DNS is a free Internet name resolution service similar to the free version of OpenDNS. Both Google DNS and OpenDNS are designed to speed up and help secure the Web browsing experience by leveraging a fast worldwide network of DNS servers.

What's the difference between these two services? As of now, OpenDNS is the more established product, having existed for four more years than Google's and offering more options for setup and control. Which one is faster, or more secure, can be debated. The primary and secondary public IP addresses of each differ, obviously:

OpenDNS: 208.67.222.222 and 208.67.220.220
Google DNS: 8.8.8.8 and 8.8.4.4

→ See also - What Is DNS?