This is a milestone in many ways – the core of Drush has been completely rewritten (some parts of it twice over!) since the last stable release. This core can now (among other things) connect to multiple major Drupal versions, work reliably (we hope!) on a wide range of command line environments – but is also a powerful platform that allows commands to specify dependencies, to work at different levels of the Drupal bootstrap, call other commands (including on remote servers over secure SSH connections) and provide responses in both machine readable formats and nicely laid out human readable formats. There are also many improvements in the set of commands that come with Drush, both adding new functionality as well as making the existing functionality more intuitive to use.
Over the next few days, I will be uploading a series of short screencasts, covering how to get started, going through the various commands in some detail, and perhaps getting into some detail about how to write your own commands.
Here is the first in the series. Please add any questions in the comments, and I will try and address them (either in the next video, or in the issue queue).