We were in LV October 27-November 1, 2013. We had never been to the buffet at Rio but decided to try it since it gets some really good reviews from the public. It was somewhat pricey (a lot of the buffets in LV are no longer the bargains they used to be) but the selection was enormous, very varied stations and almost all of the food was topnotch. The service was very efficient and refilled drinks before you could ask and also removed empty plates very quickly.
Would rate this very highly as buffets in LV go but the price was also at the upper end of what I would pay.
Noticed this trip to LV that there seems to be a trend of putting food courts in the hotels which are efficient and a lot cheaper then paying for a buffet. There are still some reasonable buffets in LV with decent food and selections (Flamingo and Harrahs buffets come to mind) so if your looking for somewhat more moderate prices go there.