Reviews can be good to get an idea of what to check out in the first place. I always download and listen to the stuff before I buy though, unless I'm absolutely sure the stuff is quality.
I agree that the reviews aren't always thrustworthy, it seems like some is more focused on licking the label's ass so they'll get more free promos than writing their honest opinion. As Basilisk said, reading the reviews sometimes make you think that every release is top class, which in fact is very far from the truth. But still, they can be to some use, especially the progressive releases, which interests me most within psy these days.