Ratings & Reviews hit Natty yesterday, and I can confirm they're working pretty well. The process is pretty much painless, the interface is straightforward and even server-side, it looks like its working pretty well.
My question is, are we going to see Ratings & Reviews cherrypicked from Natty and backported to Maverick's Software Center?
Yes.
According to the Software Centre's Ubuntu Wiki page, ratings and reviews will be released in Maverick in February 2011.