As a QQ user I'd like to note that it seems quite superior feature-wise to MSN messenger, Gtalk, Facebook messenger and Skype IM. You can for instance paste multiple images straight into the input box and send them just like text, and it also has an extremely quick and easy to use screen capture feature. Not to mention a reliable, indexed message history, offline file transfers (you send it when the other person is offline, and when they sign in they'll be able to receive the file), and accurate notification whenever a message you send fails to be delivered.

Note however that I'm referring to the International Version; the mainland Chinese version is less appealing, full of advertisements, and hides some features behind paywalls.