This is a must use for real developers. It's the kind of plugin for people who don't want to use plugins.
Matt put together an indispensable tool for extending WordPress out of the box. I too have been using a pre-release version of this plugin for a while and it does exactly what you expect it to with minimal hassle. On top of that, the syntax needed and the snippets provided make building entirely new data models an absolute breeze.
Now all we need to do is convince Matt to build a plugin which then generates a template tag to echo out any of your keys - though that's not typically what we use them for.