Many kudos to Tpt, who did most of the work in migrating our serialization code to this new format!
This release provides serializers and deserializers for the main Wikibase DataModel objects. Nearly all code is covered by unit and integration tests, and addshore has tried it successfully against the wikidata.org API. (See [0] and [1].) This means it is in good shape, though not everything has been implemented yet.
The next step for this component is to reach full feature-parity with our old serialization and deserialization code in Wikibase Lib, so this old code can be replaced. Once this has happened, we will create the 1.0 release.