For me, the Blitz Book Review at http://www.blitzreview.blogspot.com/ will have credibility only if it's honest. There's no grade inflation in the Blitz world. Not every writer gets lauded. Not every one receives five stars.
My scale is 1 to 10, and as you can see at the blog, I use all of it. If a book receives a "5," then it's average, which in today's world of literature means, not very good. But likely readable, so whether you read the book or not is up to you.
For me there are no 10's. The perfect novel has yet to be written. There are a handful of novels I'd put in the high 9's-- a few of the best classics-- which gives today's scribblers something to shoot for. There's room at the top. At the bottom also.