We are using it in my current project ... an n-tier solution with a composite application, WCF services running on load-balanced hardware and databases replicated to multiple data centers for global distribution and DR. So this is definitely an enterprise application.
My advice is really very obvious - NHibernate is not trivial, it has a lot of functionality, so take the time to study it properly. Make sure you know what it can do (for example, I recently discovered how to display columns with computed / triggers), because otherwise you will spend time doing workarounds for problems that don't actually exist.