I will say that I can appreciate books for being very well constructed and writen and I can appreciate a book for pure entertainment value.
Many bestsellers may not be the greatest literary achievements but many of them nail the entertanment value that the average reader likes. Not to mention many people would find a lot of the great literary works to be very boring and outdated.
Some may say Grisham is a hack from a literary point of view but the fact is that millions of people have read and liked his books.
There's something to be said about selling that many books.
The best way to judge a book is how much you enjoyed reading it. I could careless what some book snob says about the Zombie books I read. I like them. They're entertaining. Nobody makes any claimes of literary greatness with these zombie novels, gut a lot of readers like them.
I see no harm in reading any book so long as you are not claiming literary greatness when it's clearly not on that level.