If Twhirl is good for posting to Twitter, TweetDeck wins the prize for reading it. This is the control centre of hardcore Twitter fans, with a multicolumn display capable of holding almost anything you like.
You can group users and see their posts in one column, see your replies in another, run searches in another, see the latest public posts in another and so on.
It's perfect if you're monitoring the world for specifics or have a huge number of followers to manage, but it can be overkill if you're only following a handful of people and don't get too involved, not to mention that it's headache-inducing and tough on the eyes.
For posting, you get a straight box to type text in, and support for short URLs (via all the main sites), TwitPic, translation and the ability to instantly jump back to your most recently used hashtags.
One excellent feature is the ability to see the most popular hashtags of the moment, and if there are any you're missing.
It's the grouping tools that really make TweetDeck essential, however, and you can't do better for hardcore Twitter browsing.