IMO the library support isn't yet there for production code (e.g. no or only rudimental GUI/networking code). But if you're just using it for computation (e.g. AI or compiling or some such) it's pretty solid, and its FFI is the most straightforward I've ever seen.

FWIW I hear that these people: http://www.missioncriticalit.com/ and these people: http://www.princexml.com/ use it for production code, but I don't know much about them, beyond that Håkom Wium Lie (of CSS and Opera fame) is the Director of the latter.

I maintain a blog about Mercury's features for interested users: http://adventuresinmercury.blogspot.com/ if you're interested.