As a whole, Twitter's official mobile app is solid. The interface is sleek, and it brings with it most of the features you'd expect from a Twitter client produced in-house.
All of Twitter's features are housed in four different navigation tabs. Home is your primary Twitter feed, just as you would expect. Each time you boot up the app, it opens to your Home screen and lands on the last tweet that you read. You can tap the Home tab to quickly jump to the top. The Connect tab charts your interactions with your Twitter friends, showing your mentions and letting you know when others retweet or begin following you. Discover houses several items including relevant stories, activity stream (what you're connections are doing on Twitter), trending hashtags, suggestions for who to follow, categories, and a find-friends function. And of course, the Me tab lets you view your profile.
Sending tweets from the mobile app is even easier than doing so on the Web. Autocomplete makes mentioning friends a snap, and you can easily attach photos and location stamps in a few taps.
In addition to Twitter basics, the mobile app offers quite a few nifty features to make it easy for you to check your stream and fire tweets off while on the go. One of the newest features is Expanded Tweets. Now, tweets that contain links to Twitter's partner sites display rich content inline, right on your stream. Also, the Twitter app offers push notifications, so you can be notified right as someone you're following tweets or retweets. And with other nice touches like multiple account support, list management, and saved searches, the app gives you pretty much everything you need.
Overall, Twitter for iOS does everything you need it to. If you're a power user and are looking for features like scheduled tweets, then you might want to look into a third-party client. Otherwise, I highly recommend this download.