B-vitamins, building blocks of mental health?  Or B.S.?

There’s a great deal of misinformation on the internet regarding B-vitamins and health.  I know this because I’ve read most of it.  During the 5 years or so when I was researching how to cure my own asthma, I devoured every text I could find regarding the physiological effects of specific vitamins and minerals — especially the B-vitamins.  Large doses of B-complex or individual B-vitamins are recommended to improve mood, reduce PMS, reduce asthma symptoms, reduce the risk of various cancers, reduce the risk of heart disease, fight yeast infections, ward off mosquitoes, and prevent or cure numerous other ailments.

Which of these claims are based on reputable clinical research, and which are bullshit?