I normally get violently sick every flu season. Ive been taking 2,000 mg a day and havent gotten sick even though everyone around me has. I also take vit d and probiotics which is supposed to help but also did this the last two yrs and still got sick.
The immune benefit is a great side effect for me. I initially started taking it as I read that it helps the body during times of stessful periods. I think something to the effect of it prevents the dumping of too much cortisol into the system.
I felt radically, fundamentally better when I first started Vitamin C supplementation (which was about 5g per day and included acerola cherry powder for bioflavinoids).
On a paleo diet I seem to need far less Vitamin C, even though I eat very little fruit and small/moderate amounts of veg. Having said that, when I caught a cold a few weeks back, I supplemented Vitamin C and felt much better after each "dose".
In the last couple of years I went from supplementing 0 per day to 0.5g per day to 2-3g per day with no noticeable affect to my sense of well being at any point. Years ago I took 6-8g per day and did not notice any effect.
Over the years I have tried a lot of different supplements and really have not noticed much of an effect in most of the them (the non-stimulant variety anyway.) Magnesium citrate does seem to help me get to sleep. CoQ10 did seem to help me feel more energetic later into the day, but that might have been my imagination. It is possible that other supplements do have an effect but it develops slowly enough that I don’t really notice it.
I used to occasionally pop a vitamin C pill, maybe like twice a week. For the last few weeks, I have been drinking 1g sodium ascorbate powder in water twice a day. Within 15 minutes of drinking it, I get a pretty significant calm, relaxed feeling. I suspect this has something to do with vitamin C's effect on cortisol, as I'm pretty certain I have some slightly wacky cortisol issues going on. I'm kicking myself for not starting C supplementation months ago.