This was our first trip to Cancun. The Sun Palace has probably the best beach being on the southern end and less crowded than the big resorts towards the north end. The main pool is very small compared to other resorts we stayed at like the Mayan Palaces. The 2nd pool is not much larger than a bath tub and is more for decoration.
That was our first all-inclusive trip and definitely our last. It is a shame to be stuck eating at the same place when Cancun has so much to offer in restaurants and fun places to dine at. The breakfast and lunch buffets were adequate and very accessible. We only had a la carte dinners at their three restaurants that offer it. The food ranged from good to very good but the service was slow. However, we never had to wait more than few minutes to be seated. (We tried that at a sister resort, Cancun Palace, and the wait was 1 1/2 hours!)
If your all-inclusive package has a trip to Isla Mujeres, we do not recommend it. The private beach they take you to is a joke, and all the best activities (swim with the dolphins), you have to pay for. Might as well book it somewhere else more pleasant.
We spend one day at another sister resort, the Moon Palace. They have a huge, interconnected pool, but the beach is terrible. Overall, the Sun Palace has the best beach and is the least crowded of all the other Palace resorts.