Let me begin by saying that Head First Javascript is a good book, at least compared to any other JS books I've seen. It isn't, however, a particularly good "Head First" book.
What I mean by this is that the other Head First books I've used (XHTML & CSS, SQL, and C#) have been highly interactive, easy to use, and really got the concepts into my head. I was amazed that, after just a few days with these books, I could actually build professional-looking web pages that were rigorously standards compliant; or create complex applications in C# quickly and easily; or navigate the intricacies of building and using SQL databases. The Head First method certainly seemed to work.
So when I heard the HF people were producing a JS book, I was overjoyed. Sadly, it's been a bit of a letdown. The book smacks strongly of rushed production, lacking many of the features that makes the HF series special. For example, in the C# book, the authors take the reader through application construction in a step-by-step manner, carefully explaining everything as they go. The effect is of a very knowledgeable teacher standing over your shoulder and guiding you while you code. The reader is actively involved in every exercise, building their code from scratch. There are copies of every piece of code available for download at Head First's website, but these are merely tools for checking the reader's work.
In the Javascript book, however, much of the interactivity is missing. The book reads like a walkthrough of the code samples, with most of the user participation taking the form of pencil and paper exercises. The reader could actually complete the book without switching on her PC. Even the code solutions on the HF website seem to be just tossed up there, with the page numbers each example refers to contained in a Read Me file (the book itself just contains a general instruction to grab the code from the website, without any indication of which code).
This all sounds pretty negative, and I must confess to being somewhat disappointed with the book. That said, it's still far superior to any other JS book I've tried. By the end of Head First Javascript, I did feel that the general scope of Javascript had sunk into my brain. I had a feeling for its overall structure and the ways in which it could be applied. I know that I can now tackle a more advanced JS book and absorb the concepts much more easily.
What I don't feel is ready to sit down and write Javascript. The whole experience with this book was just too fragmentary and too lopsidedly didactic. The experience was insufficiently hands-on to enable me to become comfortable with the nitty gritty of JS scripting. When I finished HF's C#, XHTML, and SQL books, I could sit down and churn out code, with only a few O'Reilly Pocket Guides beside me to refresh my memory on syntax and structure. That's not been the case with Javascript.
Head First Javascript just seems to lack the "stickiness" of the other titles in the series.