My policy is to write reviews only of outstandingly good and outstandingly bad books, regardless of price paid. I don't review all the books that fall into the mediocre category simply because there are too many of them and because I see nothing that I can really write about them except to say they are mediocre. I defined my rating system in my article
Books: Indie eBooks Worth Reading (I).
I do give mention to nearly all books that I buy ("buy" includes free) in my semi-regular posts called On Today's Bookshelf. My most recent one is
On Today’s Bookshelf (IX).
Part of the problem is that middling books aren't focused. There are no strong points and no overwhelming weak points. Everything is middling but not so poor that one can't read the book, but not good enough to make either a character or the plot memorable.
So what would I write a review about? To write a review, something needs to stick in my mind about the book, which means that it has to be extraordinary rather than ordinary.