It works fine (not the best) for kids with no reading difficulties, but it completely lacks the understanding and the tasks that fix phonemic deficits, the actual source of most reading difficulties.
It's not entirely a bad book, but won't be of too much use for kids with reading difficulties. Since it's only a few bucks, it's not a bad investment. Just be aware of its limitations. If your kid is not developing fluent and effortless reading (not just decoding), you will need to use a method that is aware of how to fix phonemic deficits.
See my other comments in this page for more.