I have a couple of projects coming up and I am trying to decide on a persistence framework to use. I pretty much narrowed the scope down to NHibernate and Fluent NHibernate, Linq2Sql, or Entity Framework 4.0 (there really on 4.0? really?). So far here is my 'superficial' thoughts, feelings about each (note that until I actually commit to one I won't truly understand it's benefits and/or limitations):

1. NHibernate/Fluent Nhibernate
   1.  Yeas
      1. Cleanly separates persistence layer from domain layer - you can pretty much ignore the database
      2. Strong adoption amongst the 'in-crowd' - many, many Alt.Net and .NET bloggers/developers profess to it's prowess (Ayende, etc)
      3. Supposed strong community
      4. Very configurable - lots of OSS extensions, etc.
   2. Nays
      1. Lack of centralized ownership (IMO) - no single contributer/owner, lots of work outside the main trunk 
      2. Documentation is all over the place; some of it does not seem to be written clearly or lacks steps that require some amount of presupposed knowledge of NHibernate or one of it's dependencies
      3. No support from any of the larger software dev companies
      4. From the main source forge site I don't see much activity; i see more spam that has _not_ been removed than new members.
      5. First implementor syndrome?
2. Linq2Sql
   1. Yeas
      1. Super simple, limited functionality ORM
      2. Used it, know it, no surprises
      3. Tons of documentation, blog entries, SEO fodder content...everyone has had a turn 
      4. Write code while others configure...
   2. Nays
      1. Obsolete - MS is moving to Entity Framework
      2. Minimalist ORM - you end up adding additional features (such as caching) - this could be a strong point as well
      3. Poor performance with many concurrent users until you optimize (which is relatively easy: compiled queries, etc)
3. Entity Framework 4.0 
   1. Yeas
      1. Strong MS commitment (or so it seems, unless they have another MS Research project that cannibalizes on this one)
      2. Lots of current documentation
      3. Centralized community (well MS...)
      4. Seems like the have addressed the shortcomings of Entity Framework 3.0 - I won't really know until I commit to using it, the truth always comes out in the implementation
   2. Nays
      1. Previous versions of Entity Framework were horrible
      2. YAMSOSFAS - Yet another Microsoft one size fits all solution
      3. Trust - MS seems to jump from one thing to the next - outside the grokking of us mere mortals; what they support monday might be obsolete on tuesday.
      4. Personal - I always either build it myself or fall back to MS; I need some new experiances ;)
      5. I have worries about the how easy it is to break away from the MS model of horrible code generation experiences. I have seen enough MS brochureware to make me sick for life. I need real world flexibility with MY model, domain, etc. not what MS marketing deems significant.

So that is my initial, superficial perception of each persistence framework. Next up is looking into more specific and less subjective reasons for liking or disliking each ORM solution.