At first I tried Twitterific. Although it has a great design and icon, it is very limited in what it can do (basically just read and post). I have now moved on to Twitterfon, which is fully featured and free making it all the better. The app Tweetie is very popular, but it is 3 dollars to buy.
In the beginning twitter was a one sided posting software, but the users started @replying and so twitter had to add that functionality. In its evolution, twitter users seem to be inventing new conventions almost daily. Who doesn't use Retweeting, twitpic, and trends? Twitterfon adds these in an easy to use way.
Twitterfon also uses the iPhone hardware to its advantage. With a single press of a button, it can take a picture with the iPhone's camera and include it in a post with Twitpic. Since the iPhone has a powerful GPS, Twitterfon allows to view just tweets in the area. It can even search for a #hashtag in a specified area.At big events like concerts/conferences or cities, this feature can be very handy.
Of course, on top of the more advanced features, Twitterfon also handles viewing user profiles and timelines well. Tapping a tweet brings up a screen with detailed information. If there is a link in the tweet, you can visit it in Safari. Want to RT it? Well no copy/pasting is need, just press Retweet and Twitterfon does the work for you. Most useful is the ability to view the entire tweet conversation if the tweet is an @reply.
On a critical side, I would say that the fact that the program can only load 200 tweets at a time is highly annoying, especially for anyone that is following a large number of people during a high volume time. I am also not a huge fan of the interface design as scrolling through the basic timeline, it is easy to lose information.
Mobile locations aware apps on smart-phones is definitely where the future is heading and judging by how I check Twitterfon more than my email or voicemail I can say I don't really mind.
