I've always had dark circles under my eyes, and this product really shocked me with its rapid results. The first time I used it I only used it on my right eye and not on the left. By the time I had applied my mascara, I could already see a significant difference. The side with the serum looked much less puffy. I have had no reactions to this product, which is great since I have extremely sensitive skin. I used to apply a two-step concealer under my eyes, and I find that I need to use less of that now. As others stated, sometimes the roller ball doesn't move as freely as it should, so I just swipe it across the back of my hand to get it started. I love the light, relaxing fragrance.