Vitamin C megadoses were first championed by extraordinary chemist and physicist Linus Pauling, I believe, and as prescription for good health they gained by association with him. I have read conflicting assessments of whether or not they work in different articles. What's the latest scientific consensus?
Linus Pauling published a study claiming that high doses of Vitamin C can extend survival times in terminal human cancer. A weakness of this study is the selection of the control group as described in the paper
The control group was obtained by a random search of the case record index of similar patients treated by the same clinicians in Vale of Leven Hospital over the last 10 years.
This method does not ensure that the treated and the control group are properly matched. Two later fully randomized clinical studies (1, 2) conclude that high doses of Vitamin C are not effective as a cancer therapy.
It is also claimed that high doses of Vitamin C can prevent the common cold or at least shorten its duration. There is a Cochrane review stating
Long term daily supplementation with vitamin C in large doses daily does not appear to prevent colds. There appears to be a modest benefit in reducing duration of cold symptoms from ingestion of relatively high doses of vitamin C. The relation of dose to therapeutic benefit needs further exploration.
Pauling believed that large doses of vitamin C could achieve many health benefits and he was powerful and persuasive advocate of his theories. But his persuasiveness didn't sway the scientific consensus.
One early (1976) review of the idea that the vitamin protects against the common cold reported this:
A more recent American study of adult employees of the National Institutes of Health reported in 1975 found no significant prophylactic or therapeutic benefit from ascorbic acid. The same conclusion was also reached in another British double-blind trial against placebo in 295 persons.
What practical recommendations emerge from this new evidence? Major advances in treatment are usually apparent after a few well-conducted studies, and at present no strong evidence can be found to support the routine prophylactic use of ascorbic acid in well-nourished people.
More recent systematic reviews support this early conclusion. More revealing is this comment in a BMJ article about how to do proper systematic reviews (my italics):
Professor Paul Knipschild has described how Nobel prize winning biochemist Linus Pauling used selective quotes from the medical literature to “prove” his theory that vitamin C helps you live longer and feel better. When Knipschild and his colleagues searched the literature systematically for evidence for and against this hypothesis they found that, although one or two trials did strongly suggest that vitamin C could prevent the onset of the common cold, there were far more studies which did not show any beneficial effect.
This selective advocacy should make skeptics careful when looking at the evidence base as it will, most likely, be easy to find some studies showing benefits when the full weight of evidence is strongly negative.
A more recent (2002) BMJ editorial summarising reviews of the supposed benefits of supplementary vitamins says this of vitamin C:
Vitamin C is an antioxidant, and it also inhibits the formation of carcinogenic nitrosamines from dietary amines and nitrites. It might therefore be expected to have protective action against the development of cancer and cardiovascular disease. The evidence with respect to cardiovascular disease is unconvincing. The epidemiological evidence linking a high intake of vitamin C with reduced cancer incidence is confounded by the fact that the fruits and vegetables that are sources of vitamin C are also rich in a variety of other compounds that may be protective. There is a long held belief that vitamin C (perhaps in very large amounts) is protective against the common cold. A systematic review did not support this but did find some evidence of a modest benefit in reducing the duration of symptoms of colds.
I think it is safe to say that the consensus is that megadoses of the vitamin don't have clear benefits.
Paul Jaminet of Perfect Health Diet did a good review of some current science in Fighting Viral Infections by Vitamin C at Bowel Tolerance.
Recall that in animals, vitamin C synthesis rises as much as 100-fold under disease. In humans, the limit of bowel tolerance rises up to 20-fold during illness. This suggests that bowel tolerance limit is an indicator of need.
Based on Cathcart’s testimony, thousands of patients with infectious diseases have benefited from high-dose vitamin C. Although mechanisms are not well understood, vitamin C probably helps along multiple pathways.
A well-tested therapeutic strategy would be to take 4 g vitamin C every hour with water until bowel intolerance is reached. The therapy is extremely safe, and its effectiveness is usually apparent within days.
Given its safety and the ease of testing its effectiveness, why shouldn’t every seriously ill person try this therapeutic strategy? Is there any good reason NOT to try it for at least a few days to see if it has an effect?
The only recent studies he references are
 Furuya A et al. Antiviral effects of ascorbic and dehydroascorbic acids in vitro. Int J Mol Med. 2008 Oct;22(4):541-5. http://pmid.us/18813862.
 Riordan HD et al. A pilot clinical study of continuous intravenous ascorbate in terminal cancer patients. P R Health Sci J. 2005 Dec;24(4):269-76. http://pmid.us/16570523. Hoffer LJ et al. Phase I clinical trial of i.v. ascorbic acid in advanced malignancy. Ann Oncol. 2008 Nov;19(11):1969-74. http://pmid.us/18544557.
He goes on to say in the comments
Based on Cathcart’s work I rather like the idea of bowel intolerance telling me when I might be taking too much.
In the case of C, our advice is more precautionary. Deficiency is tremendously harmful, but excess is virtually harmless. In trials, the benefit may be non-existent for 95%, but 5% may see big benefits and 0% harm. In trials the 95% dilute the benefits to the 5% out of statistical significance.