I have installed Jaxer now and the first thing I tested was the E4X functionality. Seems to work great and is a very powerful tool. You do have to keep in mind that E4X code should be kept server-side as Mozilla is the only browser that knows what to do with it.
Yeah, I know that it's a Moz-only. As I said, I was using it right around the transition to 1.0, during the betas, and I had a hard time with E4X. If it works, it's wonderful, if it doesn't, it's totally opaque.