I’m concerned about the lack of any progress or unification on the AMS project, and the fact that there’s maybe three different versions each with their own tradeoffs and caveats. For example, Caching is an ugly problem to solve with later versions, but earlier version don’t cache sideloaded relationships.
What are you using instead of AMS?
I use Ember-Pouch connected to CouchDB. You can try the Iris Couch backend for free and/or run your own Apache CouchDB.
Although I agree that the lack of progress seems a little worrying AMS 0.8 has been working like a charm for me. I love using it and there’s not a lot that I’m missing currently so I just continue to use it.
JBuilder. It can be a bit verbose, but I like the full transparency. Deadly easy to partialize and cache.
I use rabl or rabl-rails. It’s little bit faster than jBuilder.
I can’t stand rabl’s DSL. 
Most of jbuilder’s slowness comes from partial rendering, I find. Wrap those in a cache statement and it blazes.
https://github.com/apotonick/roar looks pretty interesting too!
I’m using GitHub - ismasan/oat: Adapters-based API serializers with Hypermedia support for Ruby apps.. It’s lacking some features but it’s been fairly easy to augment. I added support for schema inheritance, blocking circular references, and polymorphism fairly easily. Using JSON-API (with https://github.com/kurko/ember-json-api) has several advantages as well.