The two best ADN clients I’ve used on OS X are Wedge and Kiwi. Both are fully functional apps that perform the tasks you need them to. They have their own looks though. Wedge looks a bit like Tweetie for Mac and Kiwi looks more like Hibari (kind of). You can see Wedge (left) and Kiwi (right) above. Kiwi is slightly slimmer but I prefer Wedge’s sidebar of icons. You can even lose the sidebar by hitting ⇧⌘L and using just the keyboard shortcuts.
Both apps offer Twitter crossposting, but Wedge adds Facebook to the equation as well. You can choose which accounts to post to from the compose window. I like that Wedge responds to ⌘N even when the timeline is not visible. Kiwi requires you to have a timeline open to show the compose window.