We've finally got around to putting together an online demo of Apache Isis for would-be users to quickly grok what Isis is all about. If you don't fancy reading any further but just want to play, you can find it here.
If you're still with me, you'll find that the online demo shows how Isis can dynamically generate a (human-usable) webapp and a (machine-usable) RESTful API from the same domain object model. The REST API is that defined by the Restful Objects spec.
The online demo bundles its own documentation, which shows the full source code for the domain model (all 3 classes) along with guidance on how to use Chrome (or similar) extensions to play with the REST API directly from your browser.
Feedback very welcome on the isis-dev mailing list.