I stayed at an AI in Cancun. It is called The Dreams Cancun. We stayed on the Club Level which I certainly recommend, because drinks are easier to come by, you get stellar appetizers/hor'dourves, and even better service than the standard. It was about 4500 for two people for 6 days. The liquor was actually top-shelf, and the bartenders were AWESOME. It has about 7 restaurants, but I tired of them quickly. Restaurants around Cancun cannot be ignored, neither can the nightlife, although Dreams has a club on its property. The rooms were nice, and the mini-frig was re-stocked twice a day with water, juice, Corona, Soda, and other beverages -- at no charge. They have a Spa on-site, and it has wonderful massages and treatments.
If you are spluring, I recommend Platinum Car Rental. I rented a Mercedes SLK there for several days, and driving around Cancun in your own vehicle beats the HELL out of the Cabs, although the Cabs there are all nice and air-conditioned.
The beach there is kind of small, but it very well laid out and very clean and nice. I will say that the Sushi at the restaurant by the beach was AWESOME. The girl I was with freaking loved it so much that we went twice and were not tired of it a bit. Make sure you drink lots of tequila becaues it is free, and nothing is more fun that wasting a day doing shots and lounging around on the beach.
There is lots of stuff to do in the City and in areas around Cancun (such as tours or exploring ruins), and the hotel concierges can set you up easily and get you better rates than if you book yourself. The bellmen, maids, and staff are awesome, and just a modest tip here and there gets your total priority over those that think you don't have to tip at an all-inclusive. No limit to the amount of alcohol, and if you tip the bartender from time to time you get stiffer drinks, much quicker service, and they will suggest things to do and the like. The pools there are great as well, and it was a nice mixture of younger Americans, Europeans, and some middle-aged folks.
I'd do it again in a heartbeat, and I've only heard good reviews of Dreams. You can check out reviews on them on Expedia or other travel sites.
Oh, one of the best thing is the 24 hour FREE room service. You always get a full menu, and they will bring you whatever type of wine, beer, or alcohol you desire. Extremely fast regardless of the time you call. We ate breakfast in the room just about everyday, because their breakfast is freakin' awesome. Best orange juice I ever had.
Reviews of Dreams
One of the best things about it is that you are very close to local things, and can walk or take a short taxi ride. That means you can eat at local restaurants a few times, but you can also partake in the free room service and restaurants.
No lines to speak of at Dreams for any of the restaurants, services, or room service. Just fast, friendly, efficient service. I highly recommend Club Level over regular rooms -- the difference is worth it. Oh, and like the reviews stated -- free champagne and a cold towel when you arrive.