I've been using this product twice a week, for the last five months.
Five months ago I also started to use Korean cosmetics in general, so I cannot point to this liquid and say it is a holy grail, but I also can't say it doesn't do anything to improve my skin (and my skin improved during the last five months).
So, I've noticed that it softens my face and that my skin is less oily in general (my skin type was oily before, now I think it's normal).  I've never had very visible blackheads, but I have strings of sebum on my T-zone and they are still here, but my pores seem to be a bit smaller than before. I also have pigmentation spots, and they are, more or less, the same as before I started using this product.

Before BHA, I always use COSRX low pH good morning cleansing gel and AHA/BHA clarifying toner. The idea is to lower pH, so the acid can be more effective. I usually let the acid to work on my face for 20-30 minutes and after that, my skin feels a bit oily.  

However, I will continue to use it, because there's a lot of product still left and there's no proof that it doesn't help my skin at all.