I tend to prefer positive or balanced reviews. The excessively positive reviews tend not to be too useful, though. For example, I somewhat recently read a review gushing over Mouse Guard. that was written by someone who either worked in the company or was friends with the creator of Burning Wheel. It was all over how innovative the system was and how great this version was. While it could be true, the combination of the connection of the writer and the lack of any criticisms made it completely useless as a review.
On the other hand, negative reviews can be useful. Still, I find that a large percentage of negative reviews are just rants. I can point to a number of reviews of the 4E Forgotten Realms books that just go off on how they "ruined the setting." That's not exactly useful for deciding whether the product is worth buying, especially for someone not emotionally invested in the setting.