I've never used it.  The most commonly used ORMs in C# are Entity Framework (http://msdn.microsoft.com/en-us/data/ef.aspx), Linq to SQL (http://msdn.microsoft.com/en-us/library/bb425822.aspx), and NHibernate (http://nhforge.org/).  We use Entity Framework at work.  Entity Framework works together with LINQ (not to be confused with Linq to SQL) to build SQL queries from expression trees created with LINQ.