Public DNS services like OpenDNS or Google DNS may offer more faster lookups than your ISP DNS server of your ISP but in some cases, you may get much better download speeds if you use your ISP settings.
The job of a DNS Service, like Google DNS or OpenDNS, is to translate web domain names (like google.com) into IP addresses. It is therefore important that you choose the fastest DNS Server for your computer since it is used every time you visit a new site in the browser.
When you type something in the Firefox address bar with Google Toolbar, it automatically performs a Google search if the phrase you typed doesn't match a web domain. The situation is slightly different with OpenDNS but can be reverted with a simple hack.
When you type a non-existent web domain in your web browser (e.g. gmail.coms instead of gmail.com), you are redirected to an OpenDNS search page and not the default "Address Not Found" error in your browser. This fix will change the defaul OpenDNS setting.