I have been to Cancun many times, and it is such a great place. But with the recent stuff going on in Mexico, although more in the border area, one has to be more carefuly when traveling to Cancun now. So next time you plan for your dream vacation to Cancun, just follow some commonsense precautions. Check out this post about for more information.
I would say the most important thing is to stay in well-known tourist areas. Cancun Hotel Zone is a tourist area and is completely safe. There is tight security here, and hotel owners and managers of course have a great incentive to make sure no negative publicity is ever generated here (think of people talking on the internet...).
When you go outside, stick to tried and true tourist places, and take tours. Better to take tours than venture out in a rental car on your own (you can also get lost easily!).
But all of this should not deter you from visiting some of the less known places in and around Cancun. Just make sure you know what you are doing, and go with tours and/or guides that know the area.
Here's an ezine article I wrote as well on enjoy.
Have a safe trip to Cancun!