Are you talking about the ruins in Cozumel, or Tulum (sp?), on the mainland? My parents took the tour to Tulum. Getting there will take up most of your tour time, as you have to take a ferry to the mainland and then a bus to Tolum. If you are really interested in seeing them, then it might be worth the tranportation time. If you go to Carnival's website, they should have the details listed so you can see if this is what you want to do.
I have heard that some people just love it, while others, like my parents, said it took too long for the amount of time actually spent at the ruins.