On a scale of 1-10, I think I'd put this liner at about a 7. It's good, yeah, but it's not everything I thought it would be. The formula isn't as inky black as I would have expected, one swipe produces a dark grey colour, but you need to layer the liner up to get a proper black payoff.
In regards to longevity, I can't find anything to criticise. The eyeliner sticks around all day.
The applicator really is something unlike anything else (in hindsight I should have photographed it - oops). It has the tapered tip you'd expect from a pen liner, but instead of a felt material the Tattoo Liner's applicator is much like a small, thin paintbrush. I both love and hate this. I feel like the fibres on the applicator create a smooth line very easily, but with the tapered tip being super thin I find that the very end of my eyeliner flick can sometimes look wobbly.
Would I repurchase this eyeliner? I don't think so. Although I've dabbled with my fair share of liquid pens, Stila's Stay All Day remains my all time favourite.