It provides more customization options than the alternatives, namely TweetMeme. You can use any link as a trigger for the overlay, and it’s possible to re-style the box. You can also change much of the text used, and set which short URL is placed in the tweet.
The only drawback is that users have to connect their account with the application. It only has to be once, and the user can then use any Like it? Tweet it! box around the internet. This is something that has been bothering me about the Twitter @Anywhere platform. Why should a basic tweet box or follow button widget require the authorization process, while the follow buttons in hovercards don’t? Why can’t they use the user’s Twitter login cookie like the hovercards do?