I've definitely seen fake reviews. Series of glowing ones way to good to be true. I think these often involved free samples given away by sellers and had been under the impression that Amazon had at least made some effort to curtail these.

As for reading reviews in general, I try to make a point of going through the negative ones. Some can be nonsensical eg. "item arrived late" or "wrong item supplied" but I do like to try work out where people have had a genuine problem with an item before making a purchase.