I've been in favor of up/down for years over x out of 5 or x out of 10. The core reasoning is: it captures your overall emotional response to something very succinctly. Sure you might hate X more than Y or like A more than you like B, but you'll return, revisit, recommend all the things you like and not do so for the things you don't like. When you can rate out of 5 or 10 you get thrown into a sort of tizzy trying to rationalize a system of what is 1 star, 2 stars, etc. Do you subtract a star because you think something is overpriced even though you like it? Do you subtract a star because it's hard to get it (say slow delivery) even though you like it? Basically you want to start to categorize every facet of a product/service which becomes complicated quickly. Thumbs up/down avoids all that and I think over many hundreds or thousands of reviews the ratio of up to down will provide a good probability indicator of liking something or not.