I bought this in my first (of three) experimental hauls from The Ordinary and, as you can see, I actually finished it (leftover from that batch at the moment remain my G.Acid toner and Niacinamide / Zinc). I chose this Vit C formulation originally because, I think, I did not want a silicone or a powder and I believe I read at some point that Ascorbic Acid was a 'good' version of Vit C(????). My plans with this were to target redness from acne (mostly PIH), and promote general healing.
After some experimentation, I applied this in the morning after cleansing, before moisturising, because I read that combining Vit C w/sunscreen somehow amplifies the protection from both, and because I do not like to load up my night time routine with too many actives. This lasted the equivalent of a few months of daily use.
With no control condition I cannot say for certain but I believe this product was associated with improving redness, while I was using it. It also did not cause any irritation or any other problems. I was mildly frustrated with the formulation for feeling oily, and I found it was not always easy to layer, especially under moisturiser + sunscreen + whatever make-up...I have used only one other Vit C serum (from LRP, which includes salicylic acid) so have little to compare this with. It is, obviously, a fraction of the price of most other Vitamin C serums. At the moment I have cut most things from my routine while I adapt to Differin. However, if/when I incorporate Vitamin C again, it will probably be this one. I am only about 70% happy with the experience of using it, but the price (+ efficacy) makes it a no-brainer. If I find another product that combines different ingredients so I can reduce the number of steps, I might consider that instead.