Big pharma doesn't really issue big studies into vitamins, since it would mean they'd lose a ton of money as people would make themselves well through nutrition instead of medication.

I know that vitamin c is excellent for curing colds and flu. My cousin ate a tub of chewable vitamin c when he had the flu (he was young and thought they were sweets) and his situation cleared up immediately.

Another guy claims he cured himself of cancer by overdosing on vitamin c.

Also smoking and coffee destroys vitamin c in the body (dont know a better way of putting it) so smokers and coffee drinkers should take extra vitamin c.

So yes it is good stuff. But be aware, you can actually overdose on this stuff. People say you just pee out the extra and it's no problem. But if you take too much you'll put undue stress on your liver as it has to filter it all out. So stay within reasonable limits if you are going to take more than normal.