And now from the view from Skepticville, population: me.
There's another thread on this topic on MAB where I cite the research papers on this. I'm firmly of the opinion that virtually no one healthy with a reasonably balanced diet needs supplemental vitamins. Well OK, except pregnant women - they need folic acid for sure.
With regards to performance enhancement for bodybuilders:
Over the last few years, a number of studies have shown supplemental vitamins decreasing recovery in athletes. It's not clear why this should be; possibly larger amounts disrupt regular metabolic pathways.
With regard to general health:
The Cochrane Review of the available scientific literature concludes that there is no firm evidence vitamin supplements do any good, and some evidence that they may cause harm.
Concentrate on your diet, throw your multivits in the trash and save yourself some money. If you're determined to take them, at least go for a low dosage brand, paying particular attention to the amount of beta-carotene (if it's in there, you want as low as possible).
Lifetime best: 500/363/573 @ 220 belt only
"The proper study of mankind is books" - Aldous Huxley