Editors' note: This review was updated January 28, 2015, to cover new features added in version 6.2.
The official Twitter app for iOS is great at the basics, and new features for adding video and starting private group conversations in the latest version sweeten the pot. Its reliable performance and slick design make it a good option for posting tweets and following your feed, but advanced options such as the ability to schedule tweets are still not available.
It's important to note right away that the new video and group conversation features are slow to roll out. Right now, I only have the new video features. Meanwhile, my coworker has both group conversation and video features, and another coworker still has neither. In other words, it may be some time before you get them all.
Using the Twitter app
Each time you boot up the Twitter app on your iPhone, it opens to the Home screen and lands on the last tweet that you read. Tap the Home tab to quickly jump to the top, or tap on any individual tweet to pop it out to a new screen. On this new screen, you get options to Reply, Retweet, Favorite or Share, and you can view thumbnail images or other rich content contained in the tweet. What I don't like about this system is that you have to go back to return to your timeline from an expanded tweet. I would much prefer to see expanded tweets in-line (like you can on the website), without having to pop out to a new screen each time. Also, it would've been nice to get collapsible previews of links in-line as well.
The Notifications tab charts your interactions with your Twitter friends, showing your mentions and letting you know when others retweet or begin following you. Next to that is Messages, where you can look directly at direct messages between you and other people without having to dig down to your profile. It's also where you'll be able to initiate group conversations and look at past replies.

Finally, the Me tab lets you see stats such as how many tweets you've posted, how many people you follow and how many followers you have. The Me page is also where you can see all of your previous tweets and swipe to scroll through a gallery of your previously posted photos and videos.
When you're browsing your timeline, you can view tweets from the people you follow, swipe left again to Discover new content based on what people you follow are interested in, and then one more swipe to see recent activity from all your connections.
New videos and group conversations
As I mentioned above, the latest version of Twitter is getting a slow rollout of new features that include the ability to add video to your tweets and start private group conversations.
To upload video from the tweet composition window, tap the camera icon where you usually add a photo. You'll now see a video icon you can select to start shooting video. Alternatively, you can search your iPhone library for existing videos and import them directly. The important thing to note here is that you have a 30-second limit on videos you can shoot or upload.
Discuss Twitter for iOS