I've been doing some work with NHibernate lately, and I'm pretty happy with it; it works very well over all and I haven't had any significant problems. One thing that did make it a lot easier to deal with was using Ayende's NHibernate.Generics library to deal with relations in our object model, definitely recommended if you're using NHibernate directly.

In our case, we extended Ayende's work with a tiny modification and extended his EntityList<> collection to support a very simple IBindingList implementation, which really helps a lot to support the Windows Forms databinding model.