I'm happy with my black grit, koala & girlfriend. Funnily enough, I haven't experimented with many others... I seem to be happy with what I'm using - I love the boomerang though but I barely seem to use it!
But the biggest tip I learned from my trainer is that the black grit worn down will help you work quicker in doing your infills etc than an almost new one!
Well I thought "you're barmy you are! That doesn't make sense??"
Anyway I tried it once they were a bit more worn (just before my normal 'time to chuck this' period), and realised she was right! I seem to remove any lifting much easier now and it also flushes down the nail quicker too!
It really worked for me and now I can get more use out of my files too! They are really close to dead before they go in the bin now! :lol: