Absolutely: it's easier for me to think about my own work when I see it in context (i.e., with the formatting). If I had to mouse around and select "Courier New" and the color green every time I wanted to format a code sample to get an idea of whether it looked right, I would have never finished. I had just enough formatting to tell whether things "felt right," but not so much that configuring it became a distraction.
Though what _was_ a distraction was having to stop writing to switch contents and fix bugs or add features to my editor. This was still happening when I was in the final stages of the book because any time I context-switched, I tried to spend as little time on it as possible so I could get back to work. In retrospect, maybe I should have just taken a day or two to focus on the editor and then never go back.
Fortunately, I'm still using NJEdit for other things now that the book is done, so it seems to have been worth the investment. I've also open-sourced it so others can use it or fork it for their own needs, so hopefully others can find a way to benefit from it.