Echofon for Mac is no longer in development. TUAW's Twitter Client Project surveys popular desktop alternatives in highly subjective reviews.
Itself a refugee from Twitter's somewhat developer-unfriendly new policies, the devs withdrew the original Kiwi Twitter client. They are focusing on providing a full-featured ADN client instead.
I really like the visual look of Kiwi. It's sparse and clean. The devs have made some really good layout choices. The icons are well-chosen and small touches like a global feed that can stay scrolled at the top (Prefs > General > Scrolling > Sticks to top when at top) show they listen to their users.
I did, however, find the navigation more confusing than it had to be and encountered some bugs. This is a minor quibble, however.
The buttons at the top of each screen are tied to the function being displayed. You can't just treat the app as globally tabbed, which is what I initially expected. It's a stack-based navigation browser, which keeps pushing new views.
What makes this harder is that buttons at the top keep changing. The go-back button at some point switches to a personal profile button so if you just mechanically click (I did), you end up toggling back and forth between your home screen and some bit or another of your profile and never get anywhere.
I think I would have preferred tabs overall so I wouldn't have to keep searching for the global feed which came and went by screen.
Also, when you click a... what do you call it? Not a tweet. A twapp? When you click a twapp, I'd rather it have opened up a drawer to show detail views. It bothered me that basic functionality choices came and went depending on the screen. And also that where you clicked on the twapp mattered about what view came next because there were no visual indicators that each twapp wasn't a monolithic element apart from blue links.
Kiwi experienced regular crashing during my tests, mostly when the app was idle. It's still under active development and version 3.0.1 was just released. I have not had a chance to extensively test the new version.
One of the biggest problems I had with ADN was its APIs. As far as I can tell (and please correct me if I'm wrong), you cannot search the global feed with arbitrary phrases. For this reason, Kiwi only offers searches for users, not content.
I liked how Kiwi supported image posting (a must-have feature, in my opinion, for any client) but I discovered that the lag in processing the picture through img.ly didn't mix well with typing. After requesting an image link, the client actually inserted the processed link as I was typing, so the link appeared like this. It was not what I was expecting.
In the end, Kiwi seems to offer all the core functionality you need for App.net. I found it easy to read, reply to and favorite twapps. As for ADN itself, I'm still unconvinced on that front but I'm glad I gave it a whirl.