I discuss melatonin's effects on sleep & its safety; I segue into the general benefits of sleep and the severely disrupted sleep of the modern Western world, the cost of melatonin use and the benefit (eg. enforcing regular bedtimes), followed by a basic cost-benefit analysis of melatonin concluding that the net profit is large enough to be worth giving it a try barring unusual conditions or very pessimistic safety estimates.
Full essay: http://www.gwern.net/Melatonin
Without having looked at the actual studies in detail, it seems that even several of the "no benefit" studies report multivitamins to be beneficial when one does have an otherwise unhealthy diet.
Wikipedia:
From the cited New York Times article:
(I'll keep taking my multivitamins, as my diet certainly isn't balanced.)
I take multivitamins as a backup plan of sorts so that I don't have to worry too much about whether my diet is balanced.