I have successfully implemented NHibernate on two workplaces now, and I'd have to say the biggest selling point for it is that it's really database-agnostic. On a consulting firm I worked, I had to develop an application where the companies buying it had the right to demand which database they would like to use; one wanted Oracle, another SQL Server, a third MySQL and a fourth wanted the lowest overhead possible, so we went with SQLite for it. Using any other technology would have been unacceptable at best. Although not supported by any IBM-like juggernaut, it's one the most mature ORM solutions out there, with its Java base being a whole lot older and more tested than anything Microsoft has released.
Convincing my current employer was even easier. The team was happily using *gasp* ADO.NET, and spending a whole week debugging a few queries was the norm here. So before I even proposed the idea, I ported my own war-veteran generic data access layer, so that whatever project they assigned me to, I could get a working demo running so fast they wouldn't even believe it. I did my first demo using MySQL in record time, they told me I was to use Oracle, and they literally watched me generate the Oracle database in a matter of seconds. The rest is history. :)