As with any beta version, please be careful and technical skills should be in place to uninstall or troubleshoot potential issues. During first few days of testing, no issues have been discovered and it installed cleanly.
Microsoft Security Essentials – New Pre-Release version
QUOTE: As a Microsoft Security Essentials Prerelease user, you will have the opportunity to explore and test new builds of Microsoft Security Essentials before they are publically available and provide feedback to Microsoft. Your feedback helps Microsoft to make its software and services the best that they can be. As a Microsoft Security Essentials Prerelease user, Microsoft Security Essentials updates will automatically be installed on your computer through Microsoft Updates.