Amazon also gives (or used to give) you a helpful tooltip of what they want each star to mean. I think they were: I love, I like it, It's okay, I don't like it, I hate it. I've started using it for 5-point scales and a with a slight adjustment for 10-point scales. If I would say one of those things about an item that is the rating I give.

Another good thing about Amazon's ratings is that you can see a breakdown of points. A 3-star item with 20 3-star reviews is likely to be quite different to an item with 10 1-star and 10 5-star reviews.

Although if I was a "professional" doing reviews, I would probably use a grade system of A-C and F.