I had a quick look at NHibernate Start-up guide. From my first impression, following is my feedback:
1. It is not very easy to learn,
2. Has limited support for back-end Database Connectivity,
3. Requires additional work to implement the ROM and then you may have to change the mapping made in your BO to meet the real world use case.
4. Limited support for using Stored Procedures.
5. You can do nearly the same with Typed Datasets,
6. The new LINQ feature of .NET 3.x framework I think it does the same.
I am not very happy with NHibernate.
Tarek.