I love how easy this liner is to use. I can easily draw a nice line along my lashes & either leave it a nice thick black line or smudge it with a liner brush to make it look softer. I bought the Ultra Black & was afraid that it would be too harsh on me for every day wear, but I love it. I've only been using it for a week, so I don't know about the Lash Boosting Serum claims, but it's a wonderful eyeliner. It stays on all day without smudging or smearing, but it is not even water resistant. Any tearing of the eyes or splashes of water will remove it. I love it so much that I bought the Dark Brown & the Smokey Grey, but when tested on my hand the brown & grey are so dark that I don't see much difference from my Ultra Black. Brown may look a touch warmer in tone.
I've been using this for about a month, now. I don't use it everyday, but very frequently & my lashes are looking fuller & healthier!