Pros: Easy to use
Cons: Crashes occasionally
I've tried a few of the iPhone Twitter apps (Twitteriffic, Twinkle and TwitterFon) and this is a good option. It does a great job of showing tips until you dismiss them, getting you up to speed with the app's functionality quickly. It runs great, even on EDGE, and the few ads they show in the free version aren't bothersome.
I prefer TwitterFon for its simplicity.