I use TweetDeck, on both my iPhone and desktop, for all of my tweeting. It’s also how I do the majority of by Facebooking. I like that it integrates the two, both for reading and posting. There are a couple of improvements, however, that would make it a lot more efficient.
They put an @ symbol right above the keyboard, so it’s really quick to type. But what I use even more often is the hash (#) to tag my message. To do that, I have to tap the “123” key to bring up the number keypad. Then I have to tap the “#+=” key to switch to the symbol keypad. Then I can tap “#”, but I have to tap the “ABC” key to get back to the QWERTY keyboard. That’s four taps. They should add the hash to their UI, right beside the @.