I've got one but don't get to use it much. It seems to have worked the couple times I have though. I definately prefer it over pen and paper calculations, math is my arch-nemesis. The range we use has been esimated at 150 yards by a couple different people, and the Mildot Master said 140; I assume it at least works good enough for casual shooting. I've heard it's got a military contract and is being used in Afganistan and Iraq.
You could give a new guy all the gear you want... and give an old WW2 vet an old beat up service rifle... i garrentee the old vet would win. Point being... all the cards... high end 50+ power scopes in the world wont make you any better... practice practice practice... All these cards and tools do is take away from the actual art of the craft... knowing and visualizing the bullet path as you slowly pull the trigger... and actually hitting what your aiming at without any one or anythings help.