yes and yes it can be if you are having problems with your current DNS, if your not then either don't bother...... or keep your primary DNS server as it is and use a openDNS one as the secondary. to give a better fault tolerance.
I route all my web browsing via OpenDNS, with no problems.
You only need to subscribe to the optional services. Anyone can use their domain name server for free (as I do). Those optional services can be useful if you want to filter out 'adult services' or use other filtering (such as anti-phishing) but most people either don't need them or can get similar services free from their ISP.