I've used both. JSON's quotes everywhere make it annoying to type and read. Zero benefit, just silliness. The lack of comments is also a huge pain. No easy way to document config files. No easy way to cut out part of a message. Just stupid really.

XML's fatal mistake was requiring the name in the closing tag. Had they just used <> or </ or </> people wouldn't mind as much. Also ask the stupid external entity, DTD, and association with "enterprise" terrible designs. But JSON will get there over time. Already people are pushing the equivalent of XSD and WSDL because they are useful.