I see a lot of blogs do this. My definition of good may not be the same a some one else's. So, I should try to clarify. My ratings are usually determined by my mood, sadly. I'll make an effort though.
I'll probably never give out a 5. That's supposedly perfect. And nothing ever written is perfect. When I do give out let's say a 4.75, for instance, it's probably because the book made me happy and want more. That is probably the highest I'll go. I don't even think Twilight could get that.
Again, since I'm moody, this whole scale could be invalid. I'll try to control it though.