I actually like this book a lot. The five stars are kind of irrelevant on this I feel, as these screenplay books often have different emphases, so are not better or worse, but make their points to varying degrees of success. Some of them are so Hollywood that they are useless outside that context.
'How not to' as the title suggests does give you the feeling of being slapped in the face every time you come even close to making one of the 101 common mistakes. This is good, although you may find it disheartening - obviously better to read before spending ages on writing your script. Avoiding many of these mistakes require common sense or the bare minimum of research on the part of let's say, the average writer, such as - Don't divide your screenplay into 3 acts, or People don't talk to themselves. These tips are useful although a good software package like Final Draft or Movie Magic simply makes many of them redundant - the macros in the software set you free to write to a greater extent and leave How not to on the shelf a bit more - not messing about with things like margins etc. There are also areas in which writers outside of the States may wish to depart from - such as length or where to place page numbers - both according to where the screenplay is being targeted.
However, there is a greater strength of this book which is not immediately apparent, which is a very well written and concise overview of story elements and structure. Funnily enough I didn't discover this for a long time after I got the book, as I assumed the whole thing was as the cover suggested (which goes to show 'never judge a book by ... you know the rest!) So instead of wading through and deciphering Joseph Campbell (without wanting to detract from his ground-breaking work on story myth), you can get a lot of what you need in this book, which is surprising after Flinn has spent so much time on literally being completely negative, virtually slapping the reader in the face.
Obviously read as widely as possible, How not to is a great book for the beginner or experienced screenplay writer to use as reference throughout most stages of the writing process.