I'm one of the FishCats - so I had the chance to test Glassfish v3 intensively this summer. I have some serious customers - so I didn't posted about this, because of the program's name. I don't know whether it is beneficial for a freelancer / consultant to be named a FishCat :-).
The technologies I'm interested in (Java EE, transactions, EJB) were surprisingly stable during the tests. I found some minor bugs as expected in the new stuff - especially the dynamic installation of EJB Container, Netbeans Plugin integration, log viewers, administration console etc. I filed some RFEs - it was a pleasant experience - I got immediate feedback from the engineers. Most of the bugs were already solved, the majority of the RFEs accepted. The persistence layer is stable; EclipseLink is based on TopLink which is part of Glassfish v2 and really mature (it's history started in actually in Smalltalk). I use Glassfish v2 in production already in many projects (some migrations as well), and the JPA-persistence is really stable.
In the latest Prelude version I tested (b28c) most of the problems were already fixed, it worked perfect with Netbeans 6.5 rc2 (Glassfish v3 comes already with Netbeans as well). However the EJB 3.1 support is not complete yet (one of the reasons might be the unfinished spec :-)) and there is no complete EJB 3.0 implementation available - so I neither can recommend it to my customers, nor use it in my projects.
Interestingly, there is commercial support available for Glassfish v3 Prelude already, with very clear CPU-core independent pricing. This strategy is especially interesting for Niagara CPUs - they have lot of threads to offer for one socket:-). Commercial support makes GF v3 Prelude even interesting for Mission Critical projects (after an evaluation phase, of course).
I would still expect some problems in development with classloading / OSGI kernel (it's new technology - so there have to be still some bugs), but such issues should be easily fixable. The engineering / community was extremely responsibe, at least during the program.
I'm still waiting for the "traditional" EJB 3.0/1 support and especially for embedding features - after that Prelude v3 will become the killer platform for a secret weapon, especially with the new monitoring capabilities. After the availability of full EJB 3.0 support I would immediately recommend GF v3 Prelude even for projects with the need for transaction management / concurrency / messaging (=most of the enterprise projects).