What do you look for in an XML editor?

I've been thinking about XML editors lately, because it seems whenever I write XML these days there's an unencoded ampersand (& that isn't written &) or something else that breaks the document when I load. This is really frustrating. I had been using XMLSpy when I worked at a corporation, and that program is great. But it's extremely expensive for a self-employed freelancer like myself, so I don't have it any longer. I wish I still had it so I could write a review of it, as I did like it then... Anyway, since then, I've been writing my XML either in standard text editors (thus the well-formed problem above) or in Komodo Edit (which is great, and I'm working on a review of it...). But I'm curious what other people look for in an XML editor. Keep in mind that I'm talking about an editor that is written to write XML - not a text or HTML editor like Dreamweaver or Nvu or Expression that allows you to write XML in the editor but doesn't include helpful features like checking for well-formedness and so on. What XML editors do you use? And why do you liike them? What features make them stand out for you as great editors?