If you're a web developer, you definitely owe it to yourself to read the Instant email: how we made Hotmail 10x faster post on the Inside Windows Live blog. Go ahead and read it - I can wait.
You see, with the right techniques, it's possible to provide a very, very compelling and performant experience for users. That translates into happier, more productive users.
I found it reassuring to read the post because although Hotmail doesn't use datajs, it still leverages some of the same techniques we used when we set out to develop a APIs to help improve web performance: