Iâ€™ve been using a custom validator library (that uses attributes like this one). However, seeing what the NHibernate validator provides, Iâ€™d stick with it â€“ since the validation is called on save on the dev behalf.
Ayende also shows how to pull out the errors.
As per his post, you can read the documentation on the library for further information.
As always, good stuff from Ayende!