All the information you need is on that thread on the official Minecraft forums. Updates to the editor come very soon after updates to the Minecraft client, and overall the program is well-developed. If you ever feel like adding on your own features, the C# source code is also available.
I'm very familiar with the application, so if any of you need help feel free to post here or PM me.
There's another inventory editor, except that you can change your inventory in-game with it which is far more practical.