Well, I think that Odoo itlself is using it in production, but it had been released only just 13 days ago so I think everybody who wanna used in production need to migrate their own solutions to v9. Anyone who plan to fresh start should think of use it because it's the latest version and always will be more feature complete. Also there are some modules that dissapear and others that became available in the enterprise version. On the other hand Odoo 8 is very stable at the moment and there are plenty of mature modules all over the place