I have been messing with JBoss for some weeks now and I must say, I am really impressed by this product.
Installation was as easy as extracting an archive, and just a minute later it is up and running. Configuration is a lot of fun actually - all the changes applied automagically just some seconds after saving the file. And deployment also works like a charm. Just copy the WAR, EJB-JAR, whatever - deployed. Delete it - undeployed.
All in all it appears to me like a fast, solid and very mature application server. And all for free - including the great documentation, which helped me a lot getting stated with J2EE in general and of course with JBoss AS.