This is for the restaurant, although the views at the lounge are also equally awesome although I didn't care for the crowd myself.
I went here for dinner with my Dad after an afternoon at Las Vegas Speedway for a track-day event a while back. The Rio is probably my favorite place to stay in Las Vegas for the price, although I had never been to this restaurant. This is easily in my top 5 dining experiences that I've ever been to - and that's saying something considering I live in San Francisco. I had the sea bass which was one of the best meals I can ever remember having it was one of the most unique tasting dishes as well. I've gotten sea bass quite a few places and haven't ever experienced anything close.
The price wasn't bad considering other options in Las Vegas and for the quality of food it's unbeatable. Very professional staff and the ambiance of the place is very comfortable. The view is absolutely amazing as well!
Highly recommend, there are a lot of places to check out in Las Vegas but you'd be passing up a great gem if you never checked this place out.