I've almost used up a full bottle and I actually really like this product. They don't list it on the label (annoying) but I emailed the company and they replied promptly to tell me that the vitamin c percentage is around 10% in the form of Sodium Ascorbyl Phosphate (often referred to as SAP.)
I really wanted to try a vitamin c serum with SAP as the form because it's supposed to be a lot gentler than the l-ascorbic acid form and potentially good for acne-prone skin. Additionally, I read a Japanese study posted on the skin care board in which a SAP serum (not this one, obviously) in conjunction with glycolic acid peels helped improve indented acne scarring over several months. The glycolic acid group alone had some results, but the group that used SAP serum in addition had significantly better results. So, it made me really interested in trying out a SAP-based serum for myself.