Most noticeable is the app's new design: when you first sign in, you'll see your timeline, along with icons along the top that let you view @mentions (including retweets), messages, and lists. In addition, Twitter has introduced auto-complete for usernames. Universal search has also been added: when you search, you can find tweets with the term you're looking for, tweets sent by people near your location, or people whose user names include that term. At the bottom of the new search section, you can scan your address book to find which of your friends who have chosen to be discoverable are also on Twitter.
Last but not least, the app can now be used even if you haven't signed in or don't have an account. You can view trends, browse your interests, see suggested users in several categories, and search to find out what people are saying about certain topics. You can now also sign up for Twitter from within the app.
Six months ago, Twitter for Android wasn't even among the top 10 most-used Twitter applications. In the last two months, however, usage has doubled and it's now among the top five along with twitter.com, m.twitter.com (the service's mobile website), Twitter for iPhone, and Twitter for Blackberry.