Having said all that, leaving positive feedback in return for payment is absolutely inappropriate.
As always, when extending one's hand toward another, one must exercise caution.
That's why both TMAN and I picked out words carefully as not to guarantee the absolute success of the product, merely its existence.
That being said, I believe that the number of positives that you grant should never exceed even half that of the number of negatives you have sent. Selectiveness of trust is very important.