I am setting up Organic Groups in an instance of Drupal 5.1 for a project I'm working on with a largish group of non-profit organizations.  While I've set up, configured, and managed several Drupal sites--they have all been 4.7.  I've dabbled with 5.1 a bit and it is very slick.  It probably is far better suited to non-profit use than the older version of the software.  The reason is intuitiveness. 

I've written in the past that for Arts non-profits (and non-profits in general) to adopt Web 2.0 applications heavily, the interface must be highly intuitive.  To date, many are simply to confusing.

Drupal released version 5.0 last Monday.  I guess this means I'll be upgrading this site pretty soon.  I played with the pre-release, and it was much more intuitive than 4.7 from the admin side. Slowly but surely, the contributed modules are following the upgrade path.  It will be interesting to see which ones quietly never move to the lasted version of the software.