We’re a few weeks away from full-blown production, but we do have customers using our product (maybe 20 users), and we use the JSON API. I’m not using Ember-Data though, I’m using an alternative that I wrote for our product. (I haven’t really ‘released’ it yet, as it still needs polish, but here it is). We’ve been using both to power our app since February, and we’ve had customers using it since March. I personally think the JSON API is great. It’s probably very similar to what you would design, but it has dumb ideas weeded out by the community.
A small bit of advice though: the JSON API is changing quickly, so make sure you keep up with it. The changes aren’t big, so if you keep up with them (maybe every 2 weeks), you’ll have a better API for it.
Also, if you’re worried about the adapter: don’t be. Adapters are really easy to implement (maybe 2 hours), and chances are you’ll have to customize yours anyway. Take a look at the one I wrote to see how simple it can be if you follow the spec.