The largest Mayan archeological site in the Yucatan is well worth seeing... once. While this amazing site was once home to the greatest civilation in the Americas, it is now a world renowned tourist destination.
I suggest taking a rental car and driving on your own. The roads are new and it is an easy drive from Cancun or Merida. This allows you to see everything at your own pace. When you arrive, speak to one of the many guides for hire in the main lobby. Negotiate a price for the day or an hour. These guides are well informed about the history of the area and make the experience more memorable. Expect to pay $30-40 usd for the day. Also I strogly recommend that you carry bottled water... the site is very hot and dehydration occurs rapidly.
The only 2 negatives I will mention is the number of people there every day, it gets crowded. And the number of vendors that have been allowed to set up inside the actual ruin area. Otherwise, well worth the day spent!
If you get the chance in March or Sept 21, this is a MUST SEE for the equinox celebration!!