Relax or Not?

 I have been working in XML filed for a long time, havn't worked with DTD and XSD. I haven't used Relax NG schema yet. 

I was asked by someone to take a look at Relax NG. It seems it is getting more popular (although not as popular as XSD). Here is a few high profile use case:

Relax NG is establishing a presence as a language for pivot schemas, that is, a canonical schema from which schemas in other formats are generated by automatic conversion. Examples: