I have one project using NHibernate 1.2. I know it\'s old, but I\'ve used it since I\'m working off NHibernate in Action, which targets that same version.
I started on 2.x+ so I can't say much...but I'll for sure upgrade to 3.0 once it's out of beta. Mainly because QueryOver looks like ICriteria on type safe steriods!