I started using Goodreads because I wanted some good book recommendations.  But the feature I use most often is simply browsing the books with the highest ratings.  The problem with this is immediately evident:  Highly-rated books aren't necessarily good.  But the other thing I've noticed is that highly-rated books of a specific genre aren't necessarily good if you don't like the genre.  For example, if you're into the young adult dystopian genre and happen to like all the highly-rated books, you might not be able to simply go over to the science fiction genre and immediately like all their best offerings.  In fact, you might end up liking some of the lower-rated ones, because science fiction just isn't your thing, or because the people who like science fiction might rate books based on a certain set of criteria that isn't the same across different genres. 

In conclusion, book ratings are fairly useless, as are pretty much any ratings of things based on personal opinions, which is pretty much everything.  The end.