It depends on the review and the reviewer here.
Reviews/comments/opinions of friends whose general tastes I'm familiar with always have high priority - not a guarantee that I'll share their opinion about all books (most likely not), but I have friends who tend to like the same kinds of books that I do, and then there are friends with whom my taste in books is diametrically opposite, so that works, too - if they love something, I know to stay away from that.
This also goes for people on various forums - including Amazon, and Goodreads friends (and people whose reviews I've chosen to follow there): once I've got some idea about a person's general tastes, which comes from their comments on and reviews of various books I've read (i.e. when I know what they think of the books I'm familiar with), their reviews or comments are useful.
Amazon/Goodreads/other reviews from total strangers: depends entirely on the review. "I loved it! Best book ever!" or "This sucked!" from a stranger - useless. A long, thoughtful review, with examples or reasons given, either positive or negative - useful, even if I don't know the person.