The good, according to TechCrunch: youtu.be works exclusively with YouTube videos, so it can’t be used by spammers, and because youtu.be uses YouTube video IDs, developers can mine information such as thumbnails and how a video is spreading.
The bad, according to TechCrunch: youtu.be links come out to 27 characters due to incorporating those YouTube video IDs, which are 11 characters long.
In a post on the YouTube Blog, engineering manager Vijay Karunamurthy wrote:
To see this feature in action, use AutoShare to link up your YouTube account to social networks like Twitter and Google Reader. Then, whenever you favorite a video on YouTube, for example, that action will get syndicated out to your network, who will see this shortened edition of the video’s URL.
To use youtu.be manually, simply take a URL like http://www.youtube.com/watch?v=FdeioVndUhs and replace the “http://www.youtube.com/watch?v=” with “http://youtu.be/” to get: http://youtu.be/FdeioVndUhs. Plug that shorter URL into a browser, and you’ll see that it redirects to that video.