EM, the only time Vit C is NOTICEABLY effective to me, is when I have used it for antioxidant support for chelation. I take it for prevention, of course, but I can't discern any effects of preventative dosing.
I tried various forms of Vit C. "natural" from rose-hips, straight old vitacost.com "Vit C buffered powder", esther Vit C, chewable 1000 mg tabs, etc, etc.
I noticed virtually NO DIFFERENCE between different formulations and WILDLY different costs, just so long as I kept the mg dosage the same.
I don't know if this means that the preventative effects are also the same, I just know that the anti-oxidative effects of any type/cost of Vit C are too close to distinguish between them.