So, I got a little laconic yesterday, to expand:
It is not, in my opinion, a problem if book reviewers are paid. Professional book reviewers are a vanishing breed but there are still quite a few semi-professionals out there. My opinion (and, hey, it may just be me) is that for reviews to have their intended function they must be paid for by readers. This may be directly through the purchase of magazines devoted to or including reviews, it may be indirectly through advertising revenue, but if there is financial input and it is not from readers the review immediate begins to lose integrity and serve other purposes (such as stroking the author's ego).
Now, I have reviewed for groups that charge money for 'expedited' reviews, specifically TCM and Book Pleasures. I continue to think these are basically useful and legitimate sites and if I had the time I would still be reviewing for Book Pleasures. Saying authors should not pay for reviews is not a blanket condemnation of organisations that will allow authors to pay (even though impatience is a rather silly basis for parting with one's cash when the same service is available for free a few weeks later). But I do consider that aspect of the site's overall conduct ill-advised.
A parallel might be drawn to basically legitimate publishers who do things that are not in the author's best interest, but the overall package is still legitimate and worth while (e.g. paying royalties on net, rights grabs or charging 'set up' frees for POD editions). The publisher may be exemplary and their overall conduct very beneficial to authors, but these specific practices are still not good for authors. I am, of course, giving you my altogether subjective take on things but I consider that the more you pay for a review, the less it is worth. It is likely to be given less credence and attention by readers, which defeats the entire purpose of the enterprise.
Now we all do our own analyses and decide where to draw the line. For example, one of my books appears in an advertisement at Romantic Times. Currently Romantic Times requires small presses to buy ad space to qualify for reviews. This is an indirect method of charging for reviews and so not a good idea. I would not buy an advertisement for these reasons, but if my publisher chooses to do so I am certainly not going to tell them how to conduct their business. They know more about publishing than I do and where they take out advertisements is entirely their concern.
Several things to keep in mind are that self-POD books are generally low volume. So you shouldn't spend money on things that are unlikely to earn back the money--and it is likely to make more sense to invest your time and energy in obtain reviews for free. Even if you are not bothered about making money, paying for reviews may actually leave you worse off. For example some review services also include insincere amazon reviews, general spamming and semi-literate press releases as part of the package--and bad publicity can indeed be worse than none at all.
So, if you choose to pay, make sure you are getting your money's worth. As of now I cannot think of any fee-charging review service that is likely to yield a profit on the investment.