I have been trying tweetbot throughout the day, and it is pretty nice. It seems to be really focused on the one thing that all of Tapbot's apps are focused on - user experience. I get the same "just use it" kind of feel from Tweetbot that I do with apps like Calcbot and Convertbot. You just jump in and start using it. Alot of this has to do with their design - there are limited on screen "interruptions" when you just look at the app. Only four tabs on the bottom, and the tweets (along with a small status line at top). In addition, rather than relying on buttons, they use more gestures (tap, double tap, tap and hold, swipe left, swipe right). That means you just start to move your fingers to do something, making you feel more connected to the iPhone (and by extension, the app itself). Lastly, as Leanna was talking about in her review, the sounds are very subtle. They are there, so you get a sense of response when you perform and action, but it is a very low "click" or "tap" sound, not something that immediately draws your attention.
Now, there are some things it doesn't do. I can't find anyway to click on a user and get their tweets. It doesn't do push (although they say they are working on it). While it does offer some standard options (like adding photos) it doesn't have a huge array of them. You can see photos in the tweets themselves. So for someone looking for a Twitter client that does it ALL, this is not for them.
For someone looking for a twitter client that has a fair amount of functionality and just has a good "feel" to it, they might want to give Tweetbot a try.