Sorry for not updating you guys about the "duokan" firmware.
I've been using this firmware for my KDX (US) for some weeks. It's excellent! This firmware is extremely useful if you need to read plenty of PDF files. You can re-flow the text, adjust the darkness of the letters, zoom it in any ratio, etc..
It supports DJVU files.
The support for MOBI and EPUB is there but minimal.
You can read English ebooks, but all menus are displayed in Simplified Chinese. No English. No Japanese. And there seems to be no plan for an English version.
The whole installation is easy - no need to use Whispernet. Just copy all installation files to the root directory and update the Kindle just like the official ones.
You'll then have a dual boot system (official & duokan), and you can choose which firmware to use every time you restart the Kindle. You can also uninstall it by simply deleting the installation files.