As someone who has written a number of online book reviews, I loved SF Signal's new interview/discussion "How Have Online Book Reviews Affected the Publishing World?" Among the editors and writers commenting are James Patrick Kelley, John Joseph Adams, Paul Raven, Niall Harrison, Ken of the blog Neth Space, and many more. Perhaps the best comment comes from David G. Hartwell, who says "Online reviewing at this point is a hopeful mess, rather than a hopeless one. A majority of it still has the validity of a late night bar conversation, or an offhanded phone call, blurting out undefended opinions, to which everyone is entitled. The hopeful sign is that a small portion of it is written to publishable print standards, and an even smaller portion is actually edited."
I agree with Hartwell that online reviews are only useful when they are written to print standards and actually provide context and insight into why a book either soared or sunk. A number of online venues publish reviews of this caliber; the trick, though, is that these places all have editors both screening and improving the reviews they publish. A perfect example of this is The Fix, which is an online short fiction review site run with the same due diligence as a print magazine.