First of all, people are misinterpreting the usage of "Instantly" in the title. I feel as though you couldn't have progressed past 5th grade to not understand this, but I'll explain it anyway. What "instantly" is referring to is the fact that when you master the method taught in the book, you will essentially be able to instantly conjure up the correct pitch that corresponds to the given notation. By way of elimination, you can then infer that "Instantly" does not mean that the exact moment the book is opened, the gods will bless you with the divine power of impeccable and unhesitating sight singing. This may come as a surprise, but even Instant Noodles are not in fact, "instant". For anyone that does not spend time deliberately inhaling paint fumes, this book will work just fine for you. For those of you who might argue that even trained sight singing is never technically an instant process, and that the title is therefore still misleading, that may be true, but as a title it's accurate enough for any reasonably intelligent individual. Also, it sounds better than "Sight-sing Any Melody Instantly *(Sort of, but not really)".
To address intervallic sight singing, it is flawed. The people that defend it are generally ones that started with it, and spent enough time with it that they don't want to admit their time was more or less wasted. I started with intervallic sight reading myself, but I realized it made more sense to cut my losses and move forward with a superior approach.
I'm not going to outline every reason why a tonal center approach is better, but I will provide one that I consider compelling enough to switch. Sight singing should not be about mindlessly singing the correct pitches, it should also allow you to understand the music and its patterns. As a side note, sight singing also goes hand in hand with note recognition in which the aforementioned recognition of patterns is even more vital. In any case, degrees inherently provide more information than a collection of intervals. One example is that you are fairly aware of the general frequency of which any particular degree is used. In contrast, intervals do not lend themselves to providing you with such information. By nature, intervals are not grounded within the key. Each note is really only understood by its relation to the last. Unless you are using intervals to map out degrees (which, in essence, is really just a confusing approach to the tonic center method), a note's role in the key can only be determined by following the whole string of intervals back to the beginning, or re-evaluating each note individually (both are far more time consuming than simply using the tonic center method). I'll make it clear that I do not think that intervals are insignificant. However, from degrees, the intervals are very easy to figure in your head without much thought at all; for most musicians it's an automatic process. For example, the supertonic to the dominant is always a fourth. On the other hand, when using intervals, the interval of a fourth can correspond to the relationship of 6 different pairs of degrees (considering only the diatonic notes). To figure out which pair of the six you are concerned with, you have to re-examine the music (and repeat this every other time you want to figure out the actual degrees from the limited, intervallic information you've obtained). Although I didn't originally intend to, I'll also make the point that if you screw up one note when using the intervallic method, you've already messed up the rest of the song, unless you're fortunate enough to make another mistake that effectively cancels out your first.
Ultimately, this book explains a simple concept, but does so well enough to easily justify the price.