It is true that Google DNS and Open DNS can be faster than some existing solutions or settings, like AT&T, but I would not blinded change the DNS settings without validating 1st.
How fast a DNS will resolve depends of several factors, your location is one of those. So what's fast for you, maybe is not so fast for me.
Instead, I would check available DNS servers. This link is nice:
It checks your existing DNS server settings against other local DNS servers, compare, and give you a report of which one is faster and for how much (latency in ms).