Yelp, App Store, Google, and so many more use a 5-star review system, but this idea is totally flawed and doesn't benefit the consumer as much as some think. What is different between a 3 and 4 star is also different between who is doing the review. However, when you're viewing the stars as a consumer, a 4 always beats a 3, even if the scale is different for each reviewer.
What's worthy of a 5 star review? With no key or guide, it could be anything. The app might have to be flawless for Sally to give it a 5-star review. It might need to have all the features under the sun for John. Anonymous57123 might want it to do something different, and refuses to provide a 5 star review because the "app is almost perfect, but not there yet."
On the App Store, some customers review apps with 3 stars and claim that they're missing features. Maybe it'll motivate the developers to add those features in exchange for more stars. Maybe it's a cat and mouse game. Who knows! Are you reviewing the app? The developer? The customer support? Does it have a bug free experience? Is it missing a feature? Sure you can add a comment, but then all the stars just get averaged together and that's what most people look at.
You might also find 1-star reviews on apps that might crash, or might ask for payment for future releases or additional features. This isn't uncommon in the App Store today.
5-stars don't tell the truth. There are hidden meanings and thoughts between each star.
Alternatively, there are others attempting to solve this issue.
Zappos offers multiple categories for the reviewer to rate on a 5-star scale and a more/less slider.
AirBNB offers multiple categories as well.
These are a step in the right direction. They let you know what was great and waht could be improved on. That type of information is tremendously valuable to a consumer.
One of my favorites is Hipmunk. They give you so many categories that not only help you when making a decision, but help in trusting the site itself.
What review system do you prefer?