Not really all is it? Incredibly easy to bypass.
Who are you trying to block? If its kids then yer go with OpenDNS, someone who's determined to view the sites, will find a way if you're limited to using host based blocking methods.
To do it properly with any chance of success you really need something on your network or router which combines methods like DNS checking, keyword scanning, proxy blocking etc.
A white list is the best way, could that be an option for you? i.e just allowing vists to a pre-defined list of sites?