Alexa is widely used all over the world to track the performance of websites. You can see the reach (how many people out of 1 million internet users visit your site on a certain day), the rank (how many sites are bigger than yours), and the page views per user for every site. I use it daily to check the statistics for among others game.com.cn and toodou.com.
Alexa is, however, not very accurate for Chinese websites. The way they measure the data depends heavily on how many people have installed their toolbar, and also on what kind of people (demographics) install this toolbar. In my opinion in China the people that install this English software do not correctly represent the internet population. Although generally the trend is OK, it sometimes gives very strange results. For example, for Toodou we occasionaly see a big reduction in the Alexa stats although the unique visitors keeep going up. Difficult to explain, especially to investors. But because I think it is still one of the best tools around I keep following it on a daily basis.
However, Alexa generally can be used well to compare similar sites in China. E.g. gaming sites or video sites that are only in Chinese and therefore only used in China. For this there is a nice free tool available where you can compare up to 5 websites: http://www.alexaholic.com/ Give it a try, but be warned that it can be quite addictive!