This beta is primarily to address the OmniSoftwareUpdate crash some people were encountering with 3.9.1. This issue was with submitting your system information when you had multiple display adapters. If you turned off submitting system information to avoid this, we'd greatly appreciate it if you turned it back on so we know if the crash has been fixed for everyone. To do that, go to the OmniOutliner menu -> Preferences -> Update tab -> Check the 'Include system information' box.
You can read the full release notes here or head over to the download page. If you encounter any problems with this release, please let us know at email@example.com. Thanks!