For j2ee 1.4 apps, it is probably better to stick (development and production) with 4.2.3.GA.
AS5 implements Java EE 5.
AS5 implements JEE5 but also must be J2EE1.4 compatible. I just learned that's it's a specification requirement. Our production will stay on websphere 6.0 for the moment.
Our goal is first the development environment of the developers. We need speed ;-) and a good integration with eclipse maven.