Okay, let's be clear: these pocket guides are not the most thorough things in the world -- nor are they intended to be. They're a quick overview of a given area's highlights, with a map in the back. They're a good size to stick in a pocket, backpack, or purse and not have to cart around an enormous book.
That said, if I didn't have any other guidebooks available to me besides this one, I probably would not rate it so high. I used the Lonely Planet Pocket Barcelona book last year for a brief business trip to Spain, and it was more than adequate to my needs. With a longer trip ahead of me, this book will mainly be serviceable because of the map.