Echofon for Mac is no longer in development. TUAW's Twitter Client Project surveys popular desktop alternatives in highly subjective reviews.
Hibari (US$9.99) offers a clean, visually pleasing Twitter interface. Unfortunately, as these things go (and despite its price tag), it's not feature-complete compared to other clients currently on the market.
You cannot yet load saved searches from the Twitter API, one of the most essential bits of functionality I rely on. The support page notes, "Currently, we do not sync your Hibari searches with your saved searches on Twitter.com." You cannot tweet pictures directly, as I found to my dismay.
The developer has these on her to-do list, but they're not yet part of the app.
I found basic bugs that probably shouldn't have gotten past a general release, especially for a $9.99 app. For example, you can type things into the built-in search field and the field does not automatically scroll to accommodate. Despite the blinking cursor, I kept typing and the field stayed stuck. The same goes for the "keyword block" and mute options I demonstrate in this video.
I could not search using standard Twitter Boolean logic, and I was unable to find a global feed and/or searches for it. I pinged the developer to see if I had simply missed them, but have not yet heard back.
There were also stylistic choices I wasn't fond of. You cannot hide and show the text entry field, for example. It's stuck on "always open." That takes up significant screen real estate that I'd rather use for something else. The color and font choices in the visual theme seemed to draw my eyes away from the tweets (the primary content) to the avatars (the secondary content).
The app does offer clever and worthy elements like the in-line conversation expansion. Unlike other Twitter clients, it shows these details in the same window rather than using a drawer. This can lead to a sense of "Department of Redundancy Department" as you see in this screenshot.
These are the reasons that Hibari won't be finding a permanent home on my Mac for now. Although I like the overall minimalism and the geek-friendly extensions built in (for details on those, see this write-up on advanced and hidden features), Hibari was less of a "wow" and more of an "I can't wait until they get to that."