We've been tweaking the format after receiving feedback from many people, and both the exemplar implementations as well as a variety of other implementations have made it pretty easy to use our format.
Here's the news, though: we're one step further along the standardization process.
Recently, the IANA approved our application for a media type designation for JSON API. This means you can officially serve your APIs with the type application/vnd.api+json. Neat!