I'm aware of at least two projects where NHibernate has been used successfully for a while now (2 years or more). Having a deep understanding of NHib internals is crucial, as we've had few problems that was caused by NHib under the cover actions (excessive number of queries, INSERT/DELETEs being triggered when not required, NVARCHAR/VARCHAR strings generating large number of query plans etc).

You get to know the tool after using it for a while, so to don't get into trouble you could try using tools such as NHibernate Profiler (http://nhprof.com/) , or just SQL Server Profiler to track what NHib is actually generating. This could be performed as part of the code review if you've got one in place.

Check Ayende site for problems you may face when working with NHibernate http://nhprof.com/Learn/Alerts.