A developer preview release is a release of all, or part, of the final package meant to be tested by potential early-adopters. It is meant to garner feedback on existing functionality, missing functionality, unexpected use-cases, etc. Testers are expected to have a high degree of proficiency with metadata, Java, and Javadoc, and be willing to develop a basic understanding of Spring.
A developer preview release is not a final set of APIs. Any API may change, perhaps drastically, between releases.
The current developer preview release is in production use in several applications, most notably in the UK Access Management Federation for Education and Research. It is the deployer's responsibility to make sure that production use is appropriate in their situation, bearing in mind that subsequent releases of the software will almost certainly change the API in significant ways.