Fluent NHibernate solved all my problems. I asked a similar (strangely, downvoted) question on SO: http://stackoverflow.com/questions/5522590/rapid-orm-for-pai..., which prompted me to check out Fluent NHibernate.

Before that I was using LINQ-to-SQL with a repository layer and StructureMap, but I spent way too much time writing repository methods and mapping columns manually. Do yourself a favour and try NHibernate without the XML mappings.