I was a skeptical beta tester on this product having used NHibernate for nearly 5 years and Hibernate before that. I'd never been in the middle of a project and thought "You know what I need right now is a GUI designer for NHibernate".
After testing this through the beta - I'm converted!
1. The designer solves a lot of little problems that crop up. It's not possible to construct invalid mappings and so an entire source of strange/hard to find errors is eliminated.
2. The design surface is _really_ keyboard friendly and it's easy to add fields etc to entities without clicking around.
3. Mindscape's support is phenomenal - it was often a matter of hours from me logging an issue/feature request - to being told that it was fixed.
While this doesn't yet support _every_ possible NH mapping option it does support all the common ones.
I expect that Mindscape will be responsive to requests to add features as needed by users.
The NHibernate Designer can generate fluent mappings for you as well. As an aside, it's best to ask questions in our forum just because we have more folks keeping an eye on posts there: http://www.mindscapehq.com/forums/forum/25
I hope that helps!