The app is usable but it is far from complete. The developer said in a note:
Developing for the Mac is no easy task, especially a full-featured Twitter client. However, we’ve gotten to a point where while not complete, it is useable. We’ve decided to release it as a public alpha to a) motivate us to finish faster, and b) get feedback to help us build the best Mac incarnation of Tweetbot we can make. You can love it or hate it, but rest assured it will only get much much better from here.
Tweetbot is already one of the most popular and loved Twitter client for the Apple iOS devices.