Disclosure: I am not a Twitter client user, at least not on the Mac. I tried several, from Tweetdeck to the official Twitter client, formally known as Tweetie, and also some web-based clients. But I always ended up on the Twitter website. Which is a great client on it’s own. Not a problem, because 95% of what I do is on the web. I live in Chrome and therefore if I can do things without applications I prefer that, because the web is portable and applications are not.
Being on the Mac I like to have a Twitter-client though. The main reason maybe is the integration with Growl. I cannot think of any other reason except that a native Twitter-client might have more features than the web. I heard very positive reviews of the Tweetbot for iOS so that I could not resist installing this alpha version of Tweetbot for the Mac. Because they can use your/my/our feedback the application is free for now. I will give it a real shot, both on the personal side as on the application UX side.