Glassfish is Sun s reference standard for a J2EE app server. V3 supports the new 3.1 standard. However, it is only a preview. It is currently scheduled to be released on Dec 10, 2009. Of course, it can always be dangerous to be a very early adopter in a production environment. Currently V3 doesn t support JMS or clustering, for example, but they should be in the final release.

I ve used V2 in production for about 3 years and I personally like it. The web admin console makes it very easy to manage (http://localhost:4848, admin, adminadmin), and the performance is good. Here s one example, where someone benchmarked Glassfish: Blog. Of course, you should search for more examples and your YMMV. Here s a Sun document for Glassfish to help Tomcat User.

One last thing that I would add is that Sun ships, and integrates, both Tomcat and Glassfish in their Java IDE Netbeans so you can easily switch between the two app servers to test your particular app.

GlassFish Server V3 or V2 can be used in production environments but the number of users should be less than 1500. Its not very robust and scalable during high load. If used for simple applications GF works perfectly fine, as it is the reference implementation of Java EE standards by Sun which only server to be a guide to other vendors of application servers.

For more complex and high load applications, its better to go to IBM WebSphere Application Server. That s the most robust app server I have seen in my 15 yrs of experience.

Do I use GF in production? no.

Do I find it robust? yes, but I do not tax it very hard.

Have I ever found a complete set of documentation? I think so... the GlassFish v2.1 docs and the GlassFish v3 docs (http://docs.sun.com/app/docs/prod/gf.entsvr.v3?l=en&a=view)

What do I do when GlassFish ignores the J2EE standards? I file an issue here: https://glassfish.dev.java.net/issues/

Do I use in production? Yes. (Now, using 3.0.1)

Is it robust? Yes. But my point of view is from someone that likes to follow the server s developers community and can try some tricks.

What about documentation? The official one is really good, and the developers blogs are a great plus (http://blogs.oracle.com/theaquarium/). What is maybe far from other communities, at the moment, is the collective experience material (like forums), but I think the mail lists are good enough (http://glassfish.java.net/public/mailing-lists.html).