Choosing the right GlassFish build
As you know M7 is the final, Java EE 5 compatible release for GlassFish v1. It is a production quality, deployable app server. GlassFish v1_UR1 is the follow up bug fix only release to M7. v1_UR1 starts from the same source base as M7 and adds selected bug fixes only. M7 is the current stable release. v1_UR1 has additional, limited bug fixes and those builds are pretty stable.
The next release of GlassFish is v2. The v2 release contains all bugs in v1_UR1, additional bug fixes and additional functionality (JDBCRealm and HTTP compression for example). We are still in the planning stages and will be asking for feedback soon. The v2 builds vary in level of stability but provide the lastest enhancements to the server.