Mulivitamins have been disappointing to say the least. There is little evidence,
for example, that they protect men from the big killers of heart disease and
cancer. The latest slap in the face was
a huge study of 162,000 women in the Archives of Internal
Medicne that found no correlation between taking a multivitamin and heart
disease, cancer or total mortality.  Actually, some large studies have
shown cardiovascular benefits,  but this is controversial at best.
Because of studies such as these, many of those in the traditional medical
establishment like to slam taking multivitamins as complete waste of money.
However, I feel these criticisms are overstated, because there are several areas
where multivitamins may excel for us guys. One surprising area where the evidence for
multivitamins looks good is the brain.
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Several studies have shown that
multivitamins can actually boost the IQ of children.  Furthermore, there is
a study  showing that many of the vitamins are associated with improved
memory and mental scores. I will let the abstract in this study speak for
itself: "Several significant associations (P < 0.05) were observed between
cognition and concurrent vitamin status, including better abstraction
performance with higher biochemical status and dietary intake of thiamine, riboflavin, niacin, and
folate and better visuospatial performance with higher plasma ascorbate.
Concurrent dietary protein in 1986 correlated significantly with memory scores,
and serum albumin or transferrin with memory, visuospatial, or abstraction
scores. Higher past intake of vitamins E, A, B-6, and B-12 was related to better
performance on visuospatial recall and/or abstraction tests. Use of
self-selected vitamin supplements was associated with better performance on a
difficult visuospatial test and an abstraction test".
The reasons for this are many, but suffice it to say that the brain is a huge nutrient and energy hog.
It requires, among other things, lots of the B vitamins. Plus, as people age,
they often lose GI acidity and the ability to digest certain vitamins,
including the B vitamins, and minerals. A good multivitamin, therefore,
looks like a good insurance policy for you grey matter.
There is another key area where a multivitamin may help: immunity. One
study has found that a multivitamin with around a 100% of the RDA in minerals as
well as vitamins can protect against colds and flus. Several studies have
shown that in seniors, whose vitamin and mineral digestion has often declined, a
multivitamin/mineral supplementation can help prevent infections.  This
was backed up by a recent study that showed that even those in the 50-65 age
range can get an immune boost as well. 
NOTE: Many cheap multivitamins have fillers such as Calcium that
actually slows or halts the absorption of much of what is in the multivitamin!
In addition, some of the standard multivitamins are highly compressed, which is
why you could load it as a round if they put a little powder in it, and thus do
not dissolve fully.
CAUTION #1: There is some caution in order when it comes to
multivitamins. For exammple, folic acid plays a pivotal role in the
synthesis and repair of DNA, which is why it's recommended for pregnant women.
However, it appears it can stimulate certain cancers in both men and women and there is also some
evidence that the folic acid added to vitamin supplements is much more potent
than the folate found in food. The bottom line: be careful not to consume
fortified breads and cereals in addition to a multivitamin.
CAUTION #2: There is also evidence that too much selenium can raise
cholesterol.  Men need to be careful here, because cholesterol is a risk
factor for heart disease and endothelial dysufunction, which will translate to
poor erections eventually. The study above showed that those with the
highest selenium had 8% higher cholesterol and over half of those in this
category were taking multivitamins. 
1) Arch Intern Med. 2009;169(3):294-304, "Multivitamin Use and Risk of Cancer
and Cardiovascular Disease in the Women's Health Initiative Cohorts"
Personality and Individual Differences, 1991, 12(4):351-362;
Personality and Individual Differences, 1991, 12(4):363-365
Am J Clin Nutr, 1997 Jan;65(1):20-9
5) Chandra RK. Influence of malnutrient supplement on immune responses and
infection-related illness in 50-65 year old individuals. Nutr Res 2002;22:5-11
7) Am J Clin Nutr, 2010 Nov, 92(5):1251-6, "Multivitamin use and the risk of
myocardial infarction: a population-based cohort of Swedish women"
8) American Journal of Clinical Nutrition, Aug 2008, 88(2):416-423, "Serum
selenium and serum lipids in US adults"