Well, quality too, but not whether it's a "I loved this book" or "I hated this book." A review that says how the book wasn't very good, but lists all the things it could have done better, is still helpful. Maybe you're reading along, and a reviewer says something like: "the author attempts to shape fundamentally non-human creatures (drow, for instance) into something almost human." Now, whether or not the critic likes that, if you're reading along and YOU like that, you'll go pick up the book.

See what I mean?

Reviews that are SPECIFIC are usually better than reviews that are GENERAL, which are always better than three-word, "I loved it/I hated it" reviews.

Cheers