From OpenNMS

OpenNMS, like many open-source projects, has 2 parallel development tracks:

• Stable: rock-solid and production-ready, bugfixes-only (currently the 1.8 series)
• Unstable: bugfixes and new features (currently the 1.9 series)

See version numbering scheme for details.

When the unstable release is ready, the current stable release will be deprecated and the new codebase will take its place. A new unstable is created, and development continues.

In practice, the "unstable" version is in quite heavy use by developers and other users. Usually it's pretty good, and if you want or need the latest and greatest features (see the release notes: Release Notes, and new and noteworthy for details), it's well worth a try.

If you're keen to try new features, but are sensitive to stability, just give each new "unstable" release a week or so to ensure there's no show stoppers, then give it a go.