It has, I think, one nice feature that few markups I use these days have: every node is strongly-typed, which makes things like XSLT much cleaner to implement (you can tell what the intended semantics of a thing is so you aren't left guessing or hacking it in with __metadata fields).

... but the legibility and hand-maintainability was colossally painful. Having to tag-match the closing tags even though the language semantics required that the next closing tag close the current context was an awful, awful amount of (on the keyboard) typing.