Having spent 3-4 years working with WLS and WAS (driven by customers), and "keeping an eye on JBoss", when I finally tried it for real I was amazed that I had a functional hot deployment of an existing application within 2 hours of first installation (both on Win32 and Linux).
I had that "moment of revelation" when I realised the almost obvious rationale of the JMX-based architecture. AOP in 4.x looks like yet another almost "obvious step" in the right direction that I don't see elsewhere.
The *free* "Getting Started" guide by Luke Taylor is a masterpiece of direct communication. As a result of it I bought the full docset within 2 days.
I never, ever, want to use another server (but I guess that's too much to hope).
I'm extremely pleasantly erm... gobsmacked.