Oh my gawwshhh, Holy Grail liquid eyeliner right
here. It's uberly expensive (£18 for an eyeliner, sob) but so worth it. And I'm tight. It's this innovative formula which you can read all about on that link above - they undoubtedly explain it much better than I ever could. But in a nutshell, I'll give you some good and bad bits about it.
Good points. It's absolutely, 110% waterproof. If it gets splashed, if you cry, if your eyes water, or if you swim, it'll stay put which is perfect for my eyes which run because of everything. It's also rub-proof. You have to rub it extremely hard to get it to budge at all. It's easy to remove - it's the combination of these two things - you just remove it with water (not oil: it's oil-proof) and gentle pressure. It then comes off in a few large chunks, rather than smudging everywhere. It's also really black. Not a dark grey, but proper ebony. It's excellent for sensitive eyes, because it has fewer ingredients, and it lasts a long time without getting all thick and drying out. Oh, and it's free delivery from the website!
Bad points. Well obviously, the price. It's a lot of money, but if you buy the the £2.50 trial size you get a voucher for 20% off which is great. Also, the brush is quite thick apparently - I find it okay because I like a fairly thick line but some people trim some bristles off to get a thinner line. And finally, sometimes I forget I'm wearing it (it's very light you see, you can't feel it on your eyelids at all) so if my eyes water at the corners and I rub them, obviously the flick comes right off! That's about it to be honest. This stuff is really amazing, I can't praise it enough!