Beautiful site! There is no better and incredible place to learn about the mayan culture than to visit such an amazing site. Not only are the ruins spetacular, but the ocean as well. The mayan couldn't have chosen a better place to build their city!
The ride from Cancun is a bit far, but the roads are good. If you don't want to visit many historical sites, but you feel like you must see something mayan when in Cancun, Tulum is definitely the place to go.
Don't forget to take lots of pictures, to wear sunscreen and repelent. Ah, and be prepared for the heat. It's really hot in Tulum!