I installed this tool today and it's pretty cool.  I can see the latest updates from both my Twitter and Facebook friends without having to login separately to either site.  You can tweet directly from this tool, as well as reply to tweets and send direct messages.  I don't see a way, however, to post your own Facebook status updates or to comment on your Friends updates...

I take that back...you CAN post updates to Facebook from TweetDeck (still could not find a way to respond to friends' updates though  -- you'd have to launch Facebook.com to do that).  When you click the "Tweet" button, you can select to post your update to Twitter, Facebook, or both.  Sweet!

You can also put your Twitter followers in Groups so that tweets from the same category (i.e. I created Entertainment, Business, Friends) appear separately, in the same column.  Much easier to traverse and takes the awkwardness out of this social networking tool called Twitter.  I give TweetDeck two thumbs up!!