I don not vaccinate my birds and have not yet had any problems. However, I do not take them off my property, so they have no contact with any other poultry.
After discussing it with Teri @ Ideal Poultry and my local cooperative extension agent, I chose not to vaccinate for Marek's. I chose to feed medicated chick starter instead of the coccidiosis vaccine.
I was told that mareks is for the bigger poulty farmers and those who travel with thier birds and if you have a backyard flocks no need to worry as long as they stay home and don't have any contact with some other chickens. I'm not saying this is true.. but that's what a local poulty farmer told me