It's more a matter of learning how you can use them in various ways, and also of really getting into the habit of thinking with deferreds. I've been using them for about 7 years (mainly in Python) and I'm still finding interesting new ways to think about them, new tricks, etc. [Disclaimer: I'm also one of the book's authors.] BTW, if you use AUTHD as a discount code on the O'Reilly site, you'll get 50% off the e-book price or 40% off a dead tree version.
