I've been using JEdit for a while now, and am constantly amazed at how complete it is. It's even got a plug-in to manage XSLT transformations... Not as elegant as I'd usually like, but the integration into a kick-ass editor more than makes up for it.
WIth the addition of the Project Manager plugin, it's pretty full-featured, and the somewhat sluggish Java UI is still very usable (even on a pokey TiBook 400! (one aside on the Project Manager plugin - it seems to require Java 1.4.1 to install, but if you run JEdit once under the 1.4.1 prerelease, you can install the plugin and it still works under 1.3.1...
I'll retire Optimus Prime (and TestXSLT) for now and focus on JEdit and XSLT (which uses Xalan-J etc...) since it provides some rapid turnaround between editing XML, XSL and viewing output. I've set it to generate output.html in ~/Sites/ so I can bookmark it in Safari and reload as needed.