I think a review is helpful or not, depending on *why* the person liked it/disliked it. For example, some people will give a positive review because they had a good buying experience, or like the appearance of the book/item, or like the topic of the book in general. Those are not helpful to me, even if the person put a lot of thought or wrote well.
Similarly, if someone gives a negative review because they disagree with the author, or have a low opinion of the topic, or don't like some aspect of the author's politics or religion - that does not help me, because it is not what I would be buying the book for. I have read some reviews that are very impassioned, carefully crafted and reasoned -but totally irrelevant to whether the book is an entertaining sci-fi story, or a good picture book for ages 2-4, or has useful content about some how-to topic.