I just wrapped up teaching a semester CS1 course using Zelle's book. I hope I never have to use another book besides this, because this text is simply fantastic.
This was the third version of CS1 I've taught, and the first using Python instead of C. The use of Python definitely contributed to the smashing success of this class (as did an exceptionally strong group of students), but much of the credit must go to this book.
Honestly, Zelle just nailed it. The examples are illustrative and convincing: his is one of the few books that manages to avoid the trap of silly and unreal examples that therefore provide no context for a student. His writing is crystal clear and very well organized, replete with very helpful diagrams and illustrative examples (did I mention the examples?), and he has obviously paid a lot of attention to the aspects of programming that students find most difficult.
And the exercises: wow. This is the first time I haven't felt the need to write my own (although I did anyway, because it's fun). They are fair but challenging (sometimes very), and for those of us on the teaching end, you'll be happy to know that the instructor's resources come with _complete_ sets of working solutions to all of the exercises.
Three chapters stand out in particular. First is the chapter on graphics (Ch. 5). Students love graphics, and Zelle has included a very nice wrapper on top of the TKinter library, which makes for a GUI package that students can actually use. Second, there's the final chapter that actually introduces recursion and some of the interesting algorithms from the science (searching/sorting, permutations, etc.). I had a lot of fun demonstrating the difference between sorting /usr/share/dict/words with insertion sort (about 6 days) and merge sort (about 6 seconds).
But possibly the best chapter is one I almost skipped: the chapter on software development, which is centered around a case study development of a "racquetball" simulation. At the last minute, I decided to use this chapter as the jumping off point for integrating the ideas we'd seen up to mid-term into real software development. I am convinced that this made the class.
Now there are a couple of things you might want to add as an instructor: The main one is the fact that Python is such a high-level language, with so much hand-holding built in, that I'm worried that students going on to later CS classes in other languages could get a nasty surprise. I finished up my class with a primer on languages with static type systems, in which you don't have wonderful Pythony things like string/list slicing, built-in hashtables, etc. In a second edition of this book, I'd like to see another chapter on this.
Second is a very small quibble, and really just boils down to a difference with Zelle about the order in which I like to teach this material. I ended up using every chapter in the book, but in the order 2,3,4,7,8,6,9,11,5,10,12,13. As yet another thing I love about this book, the chapters are independent enough from each other, that I was able to do this with only careful selection of the sections. Actually the book lends itself very well to alternative orderings.
In short, I simply have nothing bad to say about this book, and lots of good. Zelle hit this one out of the park. Everybody should be using it.