I'm in my mid-20s with acne-prone, combination skin, and this was the first vitamin C product that I chose to purchase after researching for a few weeks.
I introduced this product slowly into my routine - just 2-3 drops in the PM, once a week at the start since it is an active product - and I was very surprised to see results beginning only a couple weeks after introducing it. Not only did the product visibly brighten my skin (both the morning after and lasting a few days), but I began to notice some of my old acne scars/marks fading over several weeks. I'm impressed with the results that I have seen so far, even with just using it twice a week at this point.
Just to note for those introducing this for the first time: it can sting a bit if you apply it while your face is still wet/if you have breakouts (or open wounds) on your face. I also would not recommend applying this after physical exfoliation as it can cause some major irritation to freshly/over-exfoliated skin. The consistency of the product is a bit sticky, however it absorbs into the skin after waiting/by the time other skincare steps are completed. It has not caused any breakouts, although I always make sure to moisturize thoroughly after using it.
The optimal routine for me is: cleanser - waiting 10 min - TIAM My Signature C-Source - waiting 10 min - Toner/Essence/Moisturizer (usually more soothing products on the nights that I use TIAM).
Overall, an effective brightening product that has definitely helped with adding a glow to my skin and fading quite a bit of my hyperpigmentation. :)