Book Ratings - How does one judge them

Just some random thoughts. Pick up any book at a bookstall, and you find a lot of people from both the media and other eminent writers have given the book very good ratings. How far do these ratings influence one to pay for the book? Are these ratings biased in any way, or do they reflect the true stature of the book?
My guess is that the book might have gone through a lot of reviews and approvals, before its sent for publishing. So the ratings might hold some worth. Again, if every book is rated fantastic, what do readers pick from the store, apart from thier interest in a genre?

The Life of Pi

The life of Pi by Yann Markel is a book about the life in sea of a shipwrecked young man called Pi Patel. He has a man eater as his only company, and one which he has to learn to tame in order to survive the ordeals at sea.

The initial chapters of the book provide a bit of dry reading, as the author establishes the character of the protagonist. He provides an insight of the mind of the youngster, who has grown to understand the behavior and mannerisms of certain species of animals, thanks to the zoo which their family owns.

The later part of the book is what gets really exiting, as Pi learns the ways of survival the hard way. In all, an excellent and engrossing read.