Every review has bias. The very act of reviewing is a subjective practice. If there was a uniform measurable code of quality we wouldnt need reviewers.
Compare two popular Olympic Sports...say the 100 meter sprint to figure skating. Which sport has more controversy over who actually wins...why it's the on with the french judge holding up the "9.7". Conversely, despiste whatever doping scandals are out there -- you don't need a 1-10 scale to determine who wins the 100 meters.
Since every review is based in bias -- the viewer has to take a caveat emptor kind of approach to the value of each review. An exhaustive background check on the motives and mechanics of a given reviewer might succeed in ferreting about the fanboys, scam artsts and malcontents but will still leave a massive number of points of view from the many, many reviewers that remain. It's worth noting however that whether or not you RECEIVE a free copy has no bearing on a reviewers credabilty. in and of itself. It would be a more productive use of time to invetigate publishers and their criteria for who they SEND a review copy too.
Among the reviewers that remain, a case can be made that depth of experience, provides a great frame of reference for assessing relative quality. Therefore on a general level, the more you review the 'better' you are. This requires the reader of a review to at least partially share the criteria of the reviewer. This, of course, is no guarantee.
The only way for an invididual to reliaby depend on a given reviewer is to, over the course of time compare a reviewers opinion on a variety of materials to their own. On these boards for instance, I find Psion's reviews most closely match my opinions of the books I've read -- I therefore tend to pay particular attention to his review - regardless of the how, why's and wear's he obtained the original product.