Should I use Google Public DNS service for my desktop?
The pros:
A possibly speedier browsing experience because Google is actively working on mechanisms to make DNS lookups faster. The odds that your ISP is doing similar work is slim.
Better security because, again, Google is actively working on mechanisms to improve security and your ISP is using a standard release of bind that hasn't included such measures
You escape any of the various mechanisms people are using to redirect lookups that fail to ad-supported sites
The cons:
Google is doing a lot of experimental stuff and any of those experiments could actually turn out to make things slower or reduce security because they are, after all, just testing these measures.
If your PC sits at a company, there are many good things you might get as a result of using the DNS servers offered to you via DHCP which include resolution of local corporate resources that won't be advertised externally and thus not visible by Google.
Your ISP already knows about your browsing habits because they can see the actual HTTP connections. Because of this, the fact that they can view your DNS lookups doesn't add much to that picture. Google, on the other hand, isn't in any position to see your HTTP connections but by doing all your DNS lookups through them, you are presenting your browsing habits to yet another external entity. Google may not be evil, but there is a downside to sharing this private info with more people.
Before you choose, jump to your command line of choice, and do a quick ping 8.8.8.8. (8.8.8.8 is one of Google DNS's Anycasted IP addresses.)
Google's Public DNS is hosted from multiple data centers worldwide, and is a such faster than most other global services. But depending on your network uplinks, it could still have a high latency compared to your ISP's local DNS cache.
It's a guesstimate, but I'd say that if Google's DNS is less than 50 ms away, then you should consider using it, and you won't feel any slowdown compared to your ISP's cache. If Google's is 100+ ms away, then I'd go for the local ISP cache instead.