Since Linus Pauling did his initial studies and got almost religious regarding vitamin C and it's supposed cure-all, fix-all effects it was initally followed up by a number of studies that seemd to indicate many positive effects on health. Among lay persons and companies selling the stuff this view still prevails.
Anyway, what I'm getting at is that according to Ben Goldacre, author of "Bad Science", the most systematic studies carried out by the Cochrane foundation has revealed serious methodological flaws in the early studies. It was simply not foreseen that the cherry picking of Pauling could have such a devastating effect on the truthfulness of the research,
The scientific consensus now is that 1. yes, malnutrition is a bad thing and in that case it's good to take a supplement and 2. there are no positive effects associated with large doses of vitamin C (or any other vitamin for that matter), in fact the ingestion of mega-doses of vitamins is associated with excess mortality. So basically what we know is the same old same old: that humans function best within a fairly narrow band. Excess in either direction is harmful.
After centuries with scurvy afflicting malnourished sailors and artic explorer it's perhaps not surprising that lots of people felt "something had to be done". And if a little is good, maybe more is better. It was plausible.
Also bear in mind that especially in the industrialized countries there has never been such a wonderful access to fresh fruit and vegetables year round. Almost no one lacks any deficiencies in any vitamins. Where deficiencies are found it's mostly due to a voluntarily restricted diet: like eating only MacDonalds or (for other vitamins) vegetarianism and veganism. One might perhaps add alcoholism here, because the extremely excessive intake of alcohol is known to deplete vitamin C.
As others have pointed out vitamin C is an anti-oxidant that neutralizes free radicals, i.e. certain molecules damaged by oxygen. Smoking, but also excercise, creates lots of free radicals in the body.
Eating a healthy diet is in all likelihood enough, and eating fruits anti-oxidizes better than the synthethic compound. (Some fruits, however, are known to have enzymatic inhibitors, and those certain fruits can in fact make some trips more unhealthy - grapefruit notably in some cases).
I very much doubt vitamin C influences the trip in any way, and I would certainly not put much stock in user observations here as they are vulnerable to the placebo effect. Homeopathy wouldn't work without placebo for instance.
One more thing: if you really wanna test this out yourself, please do a double blind study. That is: have one person not present prepare the drug in two versions: one with just the drug, another with the drug+vitamin C and have the different pills labelled. Then have another person who knows nothing of the content distribute the non-transparent capsules to the actual users. Then over a good 100-1000 trips have those receiving the pills guess whether they got the pure or mixed pill. Then do statistical analysis on it.
Heh, I'm not just being a smartass here, but I wanted to hint at how difficult it is to actually do proper science that eliminates possible sources of error and confounding variables.
