My husband and I have had the opportunity to visit many Riu properties in the past 10 years, and recently travelled to the Riu Cancun. The resort is typical Riu: gorgeous lobby, breathtaking grounds, magnificent pools, and superb accommodations. The Riu Cancun staff was always eager to please and catered to all requests without question. The hotel was fairly new, so of course all linens and in-room amenities were spic-and-span, but do remember that you get what you pay for, so tipping goes a long way.
The restaurants were again typical Riu style, with excellent buffets with a great variety of international choices. The a la cartes are good but not great, so if variety is what you crave, stick to the buffets.
The spa is outstanding, as it offers a wide variety of treatments and all of the associates are well trained and/or certified (usually not the case in most of the five-stars we have stayed in).
There is one downfall at this hotel: because of the size and position of this resort, the sun does start to fade behind the massive building around 1pm. This was not a huge deal, just an annoyance, as chasing the sun should not be an issue in a SUN destination. The ever-disappearing sun also affected the temperature of the pools, which were frigid to say the least.
All in all, there is one thing that makes this a five-star: its location. It is smack-dab in the middle of downtown Cancun, and you can walk to all of the clubs, restaurants, and markets within 5 minutes. Not to mention, ladies, it is directly across the street from the best mall in Cancun! This is very handy for pharmacies and sundry shops.
I would highly recommend the Riu Cancun if it is a consideration for you. You will not be disappointed. And just an FYI, it has a much better beach than the Riu Palace next door.