Stayed at the Rio for a couple nights during a tradeshow that was being hosted in their convention center. The property has definitely seen its better days - just kind of old and worn down. If you sign up for the Total Rewards card you can get some smokin' rates too - so if you're on a budget that might be handy.
The rooms are all suites here, which is nice. They're quite spacious and comfortable. Overall, nothing about this place blew me away or made me want to come back. I didn't really see anything in the way of customer service - the staff all just seemed to have blank expressions and didn't really offer up any recommendations unless asked. Again, no biggie, but I just didn't feel like anyone there was very personable.
As I've mentioned before, I'm usually not too picky when it comes to hotels - if it's clean and there's a comfortable bed, I'm good to go. I suppose the Rio fit the bill. Relatively clean (though the hallways were ALWAYS littered with in-room dining remnants) and my toilet would sometimes make weird gurgling noises.
Oh and side note on the Rio - Voodoo Steakhouse and Lounge (located at the top of the Rio) are both actually really nice. If you don't stay at this hotel, it's worth it to check out the food and view one night during your trip to Sin City.