I remember on my first college trip to Europe meeting a young British doctor who said that vitamins just gave you "expensive piss." I take a daily multivitamin, a vitamin D supplement, and a magnesium supplement. I also chomp chewable vitamin C, Linus Pauling–style, whenever I'm worried about getting sick.
I know I'm deficient in D from a blood test (as are a lot of people), I can't remember now why I take magnesium but it was persuasive at the time, but maybe a multivitamin isn't worth it? I'm guessing the C is probably voodoo. But sometimes I eat really well, other times I know I eat poorly–and it seems like at least a multivitamin should be a good way to know you're keeping a baseline. But I have no idea.