I don't vaccinate, though I sometimes buy vaccinated birds without batting an eyelid. I gather some diseases may be spread by certain vaccines (i.e. some make the bird a carrier of live virus) so it's worth reading widely.
Marek's is a terrible disease, but I prefer breeding for resistance, i.e. you can end up with a Marek's-resistant flock in about 1 generation. The first generation you may get many losses, and in some vulnerable strains there might be no survivors. I prefer to buy hardy birds that have some hope of surviving these things, and if Marek's appeared tomorrow I'd deal with it by humane culling and really valuing (and breeding from) those that stay asymptomatic. I wouldn't do this with too many other diseases, but Marek's is a special case.
Good luck whatever you choose, don't let anyone tell you not vaccinating is irresponsible. There are many ways to achieve a healthy, disease-free flock. Vaccination sometimes results in a flock that carry contagious disease agents, so it isn't perfect. However it may be the only way some pet strains of chickens can survive, so by all means do it if your birds are your friends.
Just my thoughts,
Erica