My opinion is that they help. I take a multivitamin/mineral tablet every day. I know that the purists will say that one can get all these nutrients through food but, realistically, I'm not sure that you can eat all the varied types of foods to get everything from A to Z every day. Vitamins are something of an insurance policy....it covers my daily requirements even when I don't get to eat all the right types of foods during a hectic day which, by the way, is pretty much every day.
I also believe that it helps keep the immune system working at peak efficiency. Oh, I get sick once in awhile. The usual colds, bugs, and whatnot; but I rarely get lay-in-bed-wish-I-was-dead type sick. It's kind of odd that, though I'm the oldest person in the group of people that I work with, I hardly ever have called in a sick day. I'm thinking that making sure I have my vitamins, while not preventing sickness, helps me fight it off and recover much more easily.
I take a blend designed for men age fifty and over. It supposedly has stuff in there to promote the health of body parts that are of concern to men my age. I just buy the generic store brand with the thought that vitamins are vitamins and that there is no difference between store and national advertised brands. In fact, I'm pretty sure they are made by the same companies anyway.
One thing that is odd though....they list all the vitamins and minerals on the back label in super tiny print. These are suppose to be for guys over 50! Don't the packagers know that most of us are half-blind? They might be trying to slip some crack cocaine or plutonium in there for all I can tell.