I have given five star reviews, (and 4, and 3 stars). Everyone has their own idea of scale, and that is a real problem. I see it this way:
1- Dont bother
2- Eh, its okayish
3- Decent probably wont read again, but enjoyed at some level
4- An author/book to watch. Enjoyed
5- Guaranteed to follow this author and look for more books. this was great!
5 Star book as an example - Her Majesty's Wizard. Loved that, destroyed the pbook reading it over and over.Probably my all time favorite... but been 20 years since I owned it. Need to buy the ebook and try again
4 Star book as an example - Spinward Fringe - Broadcast 0 (enjoyable read, but had some serious gramar issues that made it hard in places, he has since put up a newly edited copy so that might help)
So if you know those two titles maybe that will help.