The Associated Press has an interesting article that looks at the specifics of how China's censorship of Google and the internet affects Chinese online users. They tested Google in Beijing, and report that "Recent searches for taboo topics from Beijing generally produced 'page cannot be displayed' errors. The user's browser stops working for about a minute, longer if one tries to access forbidden sites in quick succession. " As an example, they point out that doing a China Google search for "Falun Gong", the spiritual movement banned by the Chinese government, causes the browser to become unresponsive; the same search done in Hong Kong results in many links to info about Falun Gong. YouTube is blocked in China but not Hong Kong.
The article concludes that "The Great Firewall isn't an exact science, but it's meant to keep most of the sensitive content from most of the citizens most of the time."