The usefulness of this book is going to greatly depend on what sort of screenplay(s) you are intending to write, as the scope of Blake Snyder's guide is very narrow. If you are yearning to tell the story of how child abuse rips apart a family in a small, Scottish fishing village... then do not look here. It will be of no help. If, however, you are looking to write a mainstream (preferably high concept) idea then this book is, in my opinion, the best out there.
I have read 20+ Screenwriting books and for straight structural insight into the popularist Hollywood model, this is fantastic. People have questioned Synder's own track record in other reviews. That's nonsense. Great actors are not taught by screen legends but by people you've never heard of. It's the same with screenwriting. Syd Field, Robert Mckee, Chris Vogler - when's the last time you saw their names before a film? In fact Synder has more credentials than most out there.
Yes, he picks out some less than briliant examples of cinema (Legally Blonde?!) but the content here is sound and evident in much, much better films than the ones mentioned. This is just good, clear advice on how to plan and fix a particular type of script. It is absolutely not for everyone, nor should every film adhere to this model, but in the narrow (but MASSIVELY successful) market that this is aimed at, it's simply essential.