In one word: Awesome.
This Drupal distribution truly displays the beauty and power of Drupal—a dedicated team over at Development Seed spent a lot of time making a great Intranet-in-a-box solution built on top of Drupal, and even though it's a simple set of modules and an installation profile, you'd have to do a little digging to tell if an Open Atrium site even runs on Drupal!
I've just been tinkering with it for a local nonprofit I help with, and I see a lot of potential. In less than 20 minutes (from the download of the tgz file), I had two groups, eight users, and a calendar/wiki/blog system running for everyone. It's drop-dead simple! If you're tasked with building an internal communications site, or a full-fledged Intranet, this is probably one of the easiest/best (and most free) solutions out there. Check it out!