As a customer, they are often useful in narrowing down one's choices, even with a margin of error from cheating or lack of enough data. But, they are still a heck of a lot of work! Ever tried choosing a wardrobe, washing machine or laptop based on reviews? You can spend hours reading wildly different accounts, which leave you more confused than when you started.
I generally read reviews to answer simple questions which the product listing does not answer. I try to find out if there are any common problems with a product.
A good FAQ and a list of the most common problems may be a better solution than pages of reviews.