Tweetie for Mac is the Mac OS version of the iPhone app. Just like the iPhone version the interface is well polished and could of come straight from Apple themselves.
The app has full access to the Twitter service without having to pop out to the website. Atebits, the developer, have managed to do this while keeping the app small and quick.
There are two versions of the app a free one and a paid one. The free app is still a full featured version with ads every so often, although for $20 ($15 until May 4) you can get rid of the ads.
Would I recommend it, yes, I even paid for it after about 10 minutes using it.