A good start, hopefully it will get even better. (Updated)

Osfoora makes a pretty good first effort. The general functionality is solid - good conversation view, inline picture support (with more supported sites than the Twitter Mac app). It also adds TweetMarker support (very handy if you use iOS or other Twitter apps to sync timeline positions).

The UI could use some refinement - retweeting has to be accessed from the right-click menu, no search within your timeline or for users, and the icon is a bit of an eyesore.

That said, right now it offers a couple good improvements over the official Twitter app, and with some work I hope it will get even better.

Update: with version 1.1 theres now a cmd-U hotkey to jump to a users page. Still areas for improvement, but there seems to be steady development on the app, so Im very happy with my purchase.

SkiRobTO about Osfoora for Twitter, v1.1