BuddyPress.org

I’m using latest branch which will be 1.2.6 on a pretty busy site at the moment (http://xfactor-updates.com/members/) and it’s extremely stable so far.
One thing I would say is that if you are upgrading & using a custom theme, you will have to make some updates to your functions.php.
Code to display join/leave group buttons and so on, secondary activity stream avatars as well as localisation for a few JS terms will need to be added.
All required code can be found in the latest bp-themes/bp-default/functions.php