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.

I've been adding new modules to an instance of Drupal we're working with where I work. Organic Groups allows you to essentially set up private sub-sites within an existing site. It can even have its own theme (look and feel). PingVision--a company we're pretty interested in right now--mentioned it in a recent meeting. It is potentially a powerful tool in mananging projects from different clients in a single environment. Coupled with Project and some of Project's add on modules, you effectively have a bug management and project management tool.

I continue to marvel at what a versatile tool Drupal is.