I've worked with XMPP for years, and while I'm not going to give XML a glowing review, I do think most alternatives would have been worse for XMPP's use-case (extensible interoperable messaging). I also think XMPP is one of a small handful of examples of XML at its best (most of the time when I see XML used extensively in other projects, it's usually a horrific experience).

It's worth noting that XMPP uses a subset of XML (so no DTDs, comments, processing instructions, is restricted to UTF-8 only, for some examples).

Framing has been mentioned elsewhere, and it's worth noting that XMPP-over-websocket is defined, well-supported and widely deployed. The websocket layer adds framing.