I prefer psgml mode in emacs to any of the icky gui IDEs that I have seen. It doesn't have any schema support though. nxml mode for emacs has RelaxNG schema support, but it doesn't automatically insert tags, you have to start typing and complete.
But I rarely encounter people that aren't preconditioned to hate emacs so...
NetBeans has some sort of XML editing, which I haven't tried, and there are probably XML editors in all the Gnome and KDE stuff.