I used Java EE 6 before its release (at December, 10th) in various projects and needed a server for development, test and deployment :-). Glassfish v3 is the official RI, easy to install server with very good NetBeans and Eclipse integration - so it became the natural choice. I filed some bugs / features requests during the migration and development of my projects.
What really surprised me was the responsiveness of the whole engineering team. It was like chatting - I got a response in few minutes. Sometimes the solution to my problem made into the next nightly build. It is exactly the opposite experience, to what I encountered in projects with closed source application servers and "platinum contracts".
The next surprise was a nice email I got after the release. Most of the bugs were fixed!:
"We've just released GlassFish v3 final, a full Java EE 6 open source app server, and we'd like to thank you for your contribution with the following bug report(s) which were fixed in this release : - EJB 3.1 Extension is Not Visible In Updatetool On Vista
 - Module Pre-Bundling For Easier Installation in GF 3
 - Re-deployment of the same WAR with EJB 3.1 fails
 - Interceptors are ignored (works with GF v2) for no-interface session beans
 - EJB 3.1 Container Is Not Visible In the Admin Console
 - Global JNDI Name (EJB 3.1) Seems To Be Incorrect
 - Support For EJBContainer.createEJBContainer / Embeddable EJB container
 - After Clearing The Logfile, The Output remains blank
 - Sessions Beans Are Not Visible in WAR Deployment
 - Performance of Embeddable Container Dramatically Decreased
 - Undeployed EAR Is Still Visible In the Tree
 - Separation of EJB 3 Interface and Bean in Different Jars Cannot Be Deployed
 - The activation of the admin UI takes too long and happens after every restart
 - RESTful admin interface seems not to work
 - EJB Embeddable Container Doesn't Work with Web Service EJB endpoint (promoted build 70)
 - Deleted Projects Cannot Be Removed
GlassFish v3 is available for download from : http://glassfish.org
We greatly appreciate your contribution as it helped us make GlassFish better for you as well as for other users.
We're looking forward to receiving more feedback from you in the future.
thank you very much,
--The GlassFish Team"
Special thanks to the whole engineering team for be as responsive and honest under pressure - shortly before the actual release and Judy Tang the coordinator of the FishCat program. Her responses had milliseconds latency :-).