It is a great time saver if you don't want to create a new account - but using your Facebook account to log into numerous sites & apps brings privacy and security worries.
Nobody wants their friends spammed with invites or want their photos revealed because you logged into a site. Facebook updated this feature a few months ago to put many of these fears to rest.
Mashable has a detailed guide, when you reach the login screen you can click edit and choose what things you share with that website before logging in. Somethings will be required. But other things, such as likes, friends list and email address can be withheld.
It's not perfect. If somebody hacks your Facebook account they will have access to any account that you log into with your Facebook account. It's better to have an individual password and account per website.