I'm planning to dive into EJB so I'm looking for a simple EJB container.
Advantages:
Glassfish would be a good choice. Especially since its a relatively small download, starts up really fast and is always up to date with the Java EE specifications.
JBoss AS isn't really that bad either. It's also free, but is a somewhat larger download, starts up slower and not being the reference implementation means its always a little behind Glassfish. Although the Java EE edition of Eclipse bundles a WTP adaptor for JBoss AS, you'd be better off installing the separate adaptor from JBoss tools if you want incremental deployment.
I did find the JBoss community to be really helpful, especially the EJB and overall JBoss AS communities (via the JBoss forum).