I am probably the last guy to know about this, but just in case...
Apparently, you're able to debug XSLT with Visual Studio 2005 by setting an input XML file on the properties of the XSLT, setting some breakpoints as you normally would and then clicking Debug XSLT on the XML menu.
You get all the normal debug windows, the ability to step into, over and out of templates. In addition, you get to see the output as it develops in another window.
Thanks for the tip, Adrian... I suppose I can now get rid of Stylus Studio. :)