It is important to realise that this is first and foremost a book aimed to help people improve their Spanish. Thus it is not a real literature work, so if you rate it as such, it would probably net a 1 star rating, as the plot is primitive, the characters not even 2-dimensional and it is generally a bad read.
As a practice book though I think it is pretty good. It's not too long, written simply (while still exposing you to various grammar) and it's really easy to read, which I think is very important, as I was actually enjoying the process rather than struggling through it.