The app downloads and installs very quickly, taking up 7.7 MB on your drive. After launching and entering your Twitter user name and password, the app quickly fills up with tweets from the people and companies you follow. The app appears to load tweets in real-time, with no apparent lag or timed refreshes. The TUAW feed, for example, scrolled by almost too quickly to read! I stopped that by simply moving the scroll bar down from the top of the tweet list.
To view the timeline of a specific Twitter user, you just single-click on the avatar icon of that user and the chronological listing of tweets is replaced by a listing of all tweets sent by that user. When looking at the tweets for a user, you can also view mentions of that person, tweets that have been marked as a favorite, or the complete profile.
Clicking a particular tweet brings up four somewhat-transparent icons for viewing a conversation, replying, favoriting, or retweeting. What's great about this app is that you can use single-key keyboard shortcuts for most actions that you wish to perform, and even use the arrow keys to move up and down through tweets. For instance, you can scroll to a specific tweet with the up arrow, press Enter to view the tweeter's feed, and then press Esc to return to the main feed. Command-R brings up a reply window, a simple T retweets (no command key needed), and so on.
Along the left sidebar of Twitter are (at top) the avatar for your primary account, a "speech balloon" icon that lists tweets (a glowing blue dot appears next to it if there are unread tweets), an "at sign" icon designating mentions of your account, an envelope icon for direct messages, and icons for lists, profiles, and searching. Icons for other twitter accounts follow -- clicking on them reveals the same icons just described.
When Twitter is running on your Mac, the familiar bluebird icon appears in your menu bar. He's actually blue only when you have unread tweets; otherwise, he reverts to being a blackbird. Clicking that menu bar icon shows where you have unread tweets, and selecting tweets, mentions, or messages from the menu opens that feed in the Twitter app. The menu icon can also be set to hide or show the app, and can also be disabled altogether. This is done in the app preferences, which are accessed either through the Twitter menu or by clicking the "blackbird" icon at the bottom of the left sidebar.
If you're intrigued by the app, by all means get thee to the Mac App Store and install Twitter for Mac immediately. After all, it is free. Let us know what you like or dislike about the new Twitter 2.0 in the comments below.