I've never used NHibernate, but I am interested in any of the free O/R mappers out there. Scott Bellware has a nice post about his decision process to end up with NHibernate, including a few mini-reviews of other O/R mappers.

He wasn't sold on it at first (because it was an alpha), but says trying it out was "one of the best hail mary calls" he's ever made. I've been happy with the little I've done with OJB.NET, but I may have to give NHibernate a try now.