I've used oXygen for quite a while now, and I completely agree with Kurt Cagel (On XMLish Things: Oxygen).
The app is quite robust, and even better, multi-platform! It is definitely the tool of choice for XML hacks. I first chose oXygen because of its support for RelaxNG and easy transformation features.
The only problem, is that it's not the easiest tool for basic authors who just need to write documents in XML. It's not quite WYSIWYG, so I still often recommend Arbortext Epic or BlastRadius XMetal for those folks.
I think the split tabs could work a little bit better (jEdit is fantastic in this regard), but otherwise very useful. I didn't know about dragging an element from the XML and having the template and XPath already created in the XSL. I think this will be a huge timesaver!