XML Schema + constraint validation

Hello, everyone:

I'm working on a new project using XML schemas,
and we're investigating various ways to enforce
constraints.  [Though we're currently using XML
schema, we are not necessarily wedded to this
technology.]

One possibility is clix (constraint language in 
XML), which looks promising; has anyone had any
good experiences with clix?

I've also seen articles about RelaxNG, RNC, and
Schematron, as well as articles describing how
to embed schematron-based rules in RelaxNG.

Alas, my knowledge of the +/- of various approaches
for implementing contraint validation is woefully
inadequate, so I'd definitely appreciate feedback
and recommendations...

Regards,

Oswald

btw: here's a clix-based commercial tool that
seems interesting:  http://www.systemwire.com