It really depends on who is doing the paying. For years people have relied on reviews written for such magazines as the
New York Times Book Review and the
New York Review of Books. The reviewers in thier pages are paid for the reviews. But they are paid by the magazine, which also assigns the book to be reviewed. I do not see any problem with this. It assures that that the reviewer is competent.
What you are talking about, however, arises on the other end of the spectrum: an author paying for his/her book to be reviewed by someone who may or may not be qualified.
One question immediately comes to mind: How different is this from paying well-known authors or people in the academic field for cover blurbs? If you are interested in a SciFi book, does a book endorsed by David Weber stand a little higher in your list than a book without his endorsement?
As regards the paid reviewer in Ficbot's original post, my answer would be it depends. I would take the time to read a few of the reviews that have been paid for. Are they all positive 5-star reviews, or do they appear to be more objective? If the former, I would not accept reviews from this particular reviewer; if the latter, I would be inclined to accept the reviews.
I have a more fundamental problem with the reviews regardless of whether paid, solicited, unpaid, or unsolicited: Who is the reviewer and what makes them trustworthy? I feel confident about the reviewers who write for the
New York Review of Books. I know many of them and the reviews are significantly more than "I like/dislike" type reviews. That is why I subscribe to the NYRB. But who is opus941 or the person who uses the pseudonym Charles Duckens? What are their qualifications? How do I know they actually read the book? FWIW, this was something I touched upon this week on my blog, "
The Missing Ingredient: Quality Control in Indie eBooks," which discusses reviews.