Normally a pre-release is code for "ugly and a bit broken", but Beak is simplistically pretty...if lacking in differentiating features. Unlike TweetDeck, Beak sticks to a single column of updates for each section: Tweets, Mentions, Messages, Faves, Search and - a new one - Stats. Statistics are there because Beak - like Bit.ly and other URL shorteners - offers to condense URLs and track clicks within the app. Tweets in the stream can be retweeted, replied to or favorited.
There's not much emphasis on actually posting Tweets, however: perhaps to save room in the interface, the posting box requires a click to open it up.
The bottom line: Beak is feature-light but still more refined than you might expect from a pre-release. Those looking for a no-frills Twitter client for Mac should at least take a few minutes to try it out. Even so, Tweetie for Mac remains a more fully-featured Twitter app on the Mac, and will likely take the lead among Mac fans.