Duakan's firmware for kindle is an excellent complement to the official one. It has native support for Chinese fonts and epub format, and has a very good PDF reader. You can switch from the Duokan system to the original kindle system in a very easy manner (but not vice versa, obviously). The best part is that, you don't need to uninstall the system before an official firmware update.
The Duokan system is under dynamic development so far. The team collects ideas and suggestions from us users directly, and try implementing them as far as possible. It is not perfect so far, but it getting better at a very fast pace, much faster than that of the kindle official firmware.