As someone who has just switched from Linq to Sql to (Fluent) nHibernate here are a few things I've noticed.
1-Linq to Sql took so long to figure out how to do the equivalent of a join-subclass. After many modifications, I read somewhere that it is not possible. It can only map inheritance if ALL the columns are in that same table. this is great if there are a few columns, but there are tons in my case and sub classes are parents to other sub classes and so on. Why should I put them all in one table for the sake of my ORM?
2- nHibernate from experience has been robust ( sometimes too much for small quick projects ) and although familiar with it through small projects, I felt it might be too much and went the route of linq to Sql since I could generate a dbml file and be going within minutes.
3- Fluent nHibernate. Takes the best of both worlds (in my case). I can map the way I want to and have my database the way I want and not have to compromise in my domain or data models. Also one word: Automapping... icing on the cake.
I would have had to go with another ORM once I found limitations and hit a few road bumps with Linq2Sql, but Fluent nHibernate made this choice easy and I don't think I'll leave it unless something comes around that does the job even better.
So, like Rob Scott said, the question is how are you abstracting you domain => data model? and are you starting with a domain or a database? How complex are the relationships? If you have any inheritance at all I'd say just go with a richer ORM framework and save yourself the grief.
Fluent NHibernate has some of the best docs I've ever found and there is so much support / notes / blogs and resources it's self-hate to do anything less... IMO! I was up and running in < 24 hours
Oh and if your'e new to nHibernate pick up the nHibernate in Action book to help grease the wheels although there is a lot of help for that framework as well.
The best indication that a tool isn't working is when you have to WORK the tool... Linq to SQL I was customizing, reading white papers all sorts of madness and it refused to generate appropriate queries, right when I was tempted to modify my table and domain, I said let me give Fluent a whirl and I'm happy I did.
Good luck to ya.. sorry for the long response this has all been in the pas 5 or so days so I guess I'm still caught up :-)