This man has been described as the worlds most powerful critic, and it's true. Nobody, in any field, has as much infulence over his/her chosen field. People the world over, quite literally, make wines to please Parker.
Don't expect to find most of the wines in this book in your local supermarket, there are better books (Gluck, Oz Clarke) for that. But if you need to know about the worlds best wines, then there is abolutely no substitute for Parker. And by best wines, this means mainly France, also US, Italy, Australia. Don't expect too much more detail from other countries, Parker doesn't rate New Zealand much for example, apart from maybe Cloudy Bay.
So if you are looking for information on reasonably priced wines that are available in the high street, avoid this book. Most of the tastings are of vintages long since unavilable outside of an auction house.
But if you are serious about wine, then this is essential. And if you have any sort of investment at all in wine, you will no doubt have this anyway. A good review of one of your "en primeur" wines will send it's value soaring.