It's fine if you are using it for pure DNS, but you have no way to be sure that you will actually use the servers of your choice in foreign networks without testing.

Further, only Enterprise class OpenDNS service blocks anything but Conficker and IR zero-day exploits - which OpenDNS does in fact block by default without signing up and configuring a Dashboard.

The point about WiFi is that the WiFi network is easily abused by the person controlling it, and mobile users willy-nilly accept any connection they are offered.