Download this app if: you want to be selective with your tweets.
Twitsper is a portmanteau of the words Twitter and whisper. It highlights Twitsper's ability to limit the 'audience' for your tweets to a select group of followers. This selection of the intended audience can be made per each individual tweet.
Good idea — If you don't want some tweets to go to certain people, or if you only want certain people to read certain tweets, this is a handy app, especially if you don't want to keep blocking people.