I'm a big fan of Vitamin C and feel much better when taking it. In fact, it is really the only vitamin or mineral that I megadose simply because I feel the RDA / AI is much too low. I also am a cautious follower of the Pauling Theory, simply because Vitamin C does some spectacular things at a megadose level ( >= 1 gram/day). It's hard to argue with a cheap supplement that can boost nitric oxide, improve weight loss, control cortisol, lower blood pressure, decrease inflammation, protect the brain - the list goes on and on. You can check out my links on 10+ Great Reasons to Take Vitamin C and Vitamin C and Erectile Dysfunction for more information.
CAUTION: In my link on Garlic and Erectile Dysfunction, I discuss how Vitamin C and garlic coupled together can dramatically increase nitric oxide and lower blood pressure. However, one should always consult with your doctor first if you have any medical condition or are on any medications. For example, lowering your blood pressure too much can have serious consequences.
In spite of all the praise I have for Vitamin C, a few cautions are in order. In my opinion, Vitamin C's track record is excellent, but a few potential "dangers" have been uncovered as well and I cover those below:
1. Blood Flow Reduction. One animal study showed that in some cases Vitamin C could possibly contribute to a loss of blood flow. This study basically demonstrated that Vitamin C could increase a key oxidant, peroxide, that is responsible for supplying blood flow to the muscles and expanding arteries.  In my link on Antioxidants and the Heart I discuss how free radicals, supposedly the villains, actually play crucial roles in our health. For example, nitric oxide is a free radical and dilates our arteries. Researchers are concerned that taking too many antioxidants, such as Vitamin C, could quench the body's requirement for these free radicals.
Now the rebuttal to this is that this study actually combined two antioxidants and thus may not really be a realistic scenario. In fact, as mentioned above, researchers have found that combining garlic and Vitamin C increases nitric oxide by three times! We have had guy on The Peak Testosterone Forum use this to cure their erectile dysfunction - that's how powerful it for some men. Nevertheless, it does beg the question as to whether we fully understand the complete impact of megadosing Vitamin C.
2. Loss of Exercise Benefits. Another disturbing study showed that 1000 mg daily of Vitamin C coupled with 235 mg daily of Vitamin E lowered key benefits of exercise. The researchers summarized by saying that "daily vitamin C and E supplementation attenuated increases in markers of mitochondrial biogenesis following endurance training."  Of course, some would argue that Vitamin C alone may not do the same and so it is too bad that the study did not also look at each supplement separately. Again, though, this shows that one can have too much of a good thing.
3. Increase Arterial Plaque?. And then came the real black eye: one study showed increased arterial thickness from Vitamin C. This study reported that those who took Vitamin C had 2.5 times the thickening of the arteries compared with those who took none.  This study has been criticized for being relatively small and one can't help but ask the question, "If Vitamin C causes such accelerated hardening of the arteries, why aren't its proponents dropping dead like flies?" Personally, I doubt that Vitamin C causes arterial thickness as this was a small and controversial study, and there are many studies showing that Vitamin C improves arterial health.
4. Increasing Nitrosamines. One can't help but wonder why Vitamin C does not fight cancer more effectively. After all, it is one of our body's most important antioxidants and it is used extensively throughout the immune system. It only stands to reason that it should help with cancer outcomes. The reason is very likely that Vitamin C very likely has a dark side: the journal Gut simulated conditions in the stomach and found that Vitamin C, when fat levels are low, actually inhibits the carcinogenic nitrosamine production but promotes nitrosamines when fat is present.  I'm not talking about fat as in overweight: I'm talking about fat in your meal. If you eat any fat in your meal - which almost every one in a modern society does - megadosing with Vitamin C will lead to a flood of carcinogenic nitrosamines and this may explain some of Vitamin C's questionable cancer results.
By the way, did you know that ascorbic acid is sometimes added to meats to lower nitrosamine production. The point is that there is data that shows the opposite of the above. As always, it's too bad there isn't more study work.
5. Raising Iron Absorption. Dr. Russell Blaylock and other recommend taking Vitamin C (in a "salt" or "Ester-C" form) in order to make sure that you do not increase iron absorption. Unfortunately, most men in Western societies consume too much iron and this associated with cardiovascular and neurological disease. Again, an easy solution is to take it on an empty stomach but not all men know about this.
6. Heart Disease. Let's start by looking at Vitamin C and heart disease, which is the #1 killer of men and the underlying cause of most erectile dysfunction issues. Here the studies have been decidedly mixed and some have wondered if this means Vitamin C has some hidden negative property that masks some of the good ones. In its favor, Vitamin C, and Vitamin E as well, were examined in a controlled study by Danish researchers in 2000. Participants were given 250 mg and 91 mg twice daily of Vitamins C and E, respectively. This study found that those on the Vitamins C and E had decreased arterial thickness, i.e. less plaque buildup from arteriosclerosis.  The researchers commented that "taking either vitamin separately also resulted in a reduction in plaque buildup but the combination of vitamins worked best..." Similarly, a large meta-analysis of nine cohorts found that those who took supplemental Vitamin C at megadose levels (> 700 mg) had significantly reduced (25%) cardiovascular issues compared to those who did not take Vitamin C. 
So everything sounds like a "green light", right? Well, unfortunately, there have been a number of studies that have found that Vitamin C has no significant heart benefit. One of the biggest was The Physicians Health Study II of 14,000 male physicians run by a Harvard professor. In this study physicians were put on 500 mg Vitamin C and 400 IU Vitamin E daily. The researchers found that neither vitamin decreased cardiovascular events whatsoever.  Then came a study on postmenopausal women with diabetes that showed an increased cardiovascular risk with Vitamin C usage.  Critics point that 500 mg of Vitamin C is a fairly low dosage according to Pauling Theory.
Another researcher, Rory Collins, looked at not just heart disease during a huge five year study involving 20,000 people who took daily 600mg of vitamin E, 250mg of vitamin C and 20mg of beta-carotene. This study  in the prestigious journal Lancet found that there was no improvement in heart disease or cancer rates. Once again, Vitamin C looked like a questionable use of one's money. Again, some have criticized this study for only supplementing with 250 mg of Vitamin, but keep in mind that is roughly the Vitamin C found in five oranges.
1) Journal of Internal Medicine, 2000; 248 (5), 377-386
2) American Journal of Clinical Nutrition, Dec 2004, 80(6):1508-1520, December 2004 Antioxidant vitamins and coronary heart disease risk: a pooled analysis of 9 cohorts Paul Knekt, et al
3) Reported on March 2, 2000, by Dr. James Dwyer at an American Heart Association meeting in San Diego
7) The Lancet, 2002, 360(9347):1781-1782
8) Gut, 2007; 56:1678-1684, Published Online First: 4 September 2007, "Fat transforms ascorbic acid from inhibiting to promoting acid-catalysed N-nitrosation"
9) J Am Coll Nutr, 2005 Jun, 24(3):158-65
15) Experimental Physiology, Published Online: 21 Jul 2009, 94(9):961-971, "The effects of antioxidants on microvascular oxygenation and blood flow in skeletal muscle of young rats"
16) Am J Clin Nutr, Nov 2004, 80(5):1194-1200, "Does supplemental vitamin C increase cardiovascular disease risk in women with diabetes?"
17) JAMA, 2008 Nov 12, 300(18):2123-33, "Vitamins E and C in the prevention of cardiovascular disease in men: the Physicians' Health Study II randomized controlled trial.
18) J of Physiology, Published online before print Feb 3 2014, "Vitamin C and E supplementation hampers cellular adaptation to endurance training in humans: a double-blind randomized controlled trial"