I visited Las Vegas in the off-season (February) and found that it's one of the best times to go. It's not crowded and a lot easier to get around through the crowds both in the hotels and on the streets.
Rio was a decent hotel, though I would probably try to stay elsewhere in the future. The beds are comfy, the towels nice and full, and the water pressure is so good it almost shoots you through the wall like Kramer in that one episode of Seinfeld. The room service was tricky though, as the three times we called down for it they were operating on a 1-2 hour wait. We only ate at the Grand Buffet and the American Grille. Both places were just ok for where and what they are. Nothing great.
The pool area was lovely for lying in the sunshine, but I imagine gets crowded with a full hotel. The desk and bell staff were all very friendly and helpful, as were most of the food servers. Every single person greeted me with a 'hello' as I passed them in the halls, from the housekeeping staff to the bellmen and room service servers.
Everything is expensive, and that's a bummer (but I guess it's like that in a lot of the Strip area). $14 for an Amstel Light and a Bloody Mary, for example.