I don't use their reviews to decide what to read, but I have checked after the fact on books I like and I think the quality of what they surface tends to be pretty bad.

A lot of mindless criticism, especially. It's perfectly OK to be critical when a book has flaws, but so many of the top reviews were people who just weren't the target audience criticizing it for being targeted at something different than they wanted. Whether that's rigorous academic nonfiction with reviews complaining that it cites its sources, kid/YA books with people complaining that there isn't enough depth, someone like Janet Evanovich or Jana Deleon writing deliberately nonsensical stuff for light humor getting complaints about not being realistic, romantic suspense getting criticism because characters are emotionally connected too fast when that's part of what the genre is, etc.

It's perfectly fine to be disinterested in a book because you're not interested in that genre, but it seems like way too many of the higher visibility reviews are people who just aren't interested in what the book is trying to do.