Thirded! 
One can accumulate quite a collection in Calibre, especially with so many free books available. And when searching the collection for something to read, it would be very helpful to see how many people have rated the books, a factor equally as important as the overall ratings themselves imo.
5 stars is great, but knowing 30 people have rated it as opposed to just one is extremely helpful.
In a perfect world (not to suggest Calibre is less than perfect as-is
), there would be an option to download/update ratings, including amount of reviews. Since some can change quite a bit, ie if a book was newly released when the calibre user added it to one's collection, so being able to conveniently update just the ratings and number of reviewers would be a very helpful asset.
Another small thought - I use a firefox extension, Greasemonkey, with a script installed called
Better Amazon. Somehow, when browsing Amazon, the script embeds the actual numerical review rating average to the hundredths. Instead of just seeing 4 1/2 stars, it also shows, for example, "4.57" next to it. I don't know how it does this (either requesting it from amazon, or calculating it), but having this information in calibre might also be an asset.