Climbed them in 1990 and again in 1992. Very cool place. I remember pottery shards littering the ground and a really bad resturaunt at the visitor's center. Montezuma's revenge...
I've always wanted to climb them, though Palenque (a Mayan site) is the one site in all Latin America I would most like to visit. I suppose it doesn't have to be an either/or thing though.