I am a little ashamed to say that I have never used an ORM; as you may recall most of my career experience is hacking around with Classic ASP and the little .NET I do tends
SubSonic from Rob Connery is a "Swiss Army knife" ORM based on ActiveRecord. You can be productive with it in 30 minutes. Be warned, it is NOT at the level of NHibernate in terms configuration options, etc. It is hugely productive, and the code is very clean. We use it on most of our projects.