This comes up every time we discuss the subject of cotters. Someone recommends the hammer. Someone else recommend the BikeSmith tool. Someone else reluctantly points out the shortcomings of the BikeSmith tool. That last one is usually me.
Look, the BikeSmith tool is great for the cotters on your garden variety three speed, especially one with a chain guard. But the finer cranks that came on lightweight bikes, with no chain guard, are both thinner and more delicate. I have two Williams cranks, and one marked Constrictor, on which the cotter is too close to the spider to fit the Bikesmith till. On these I use the modified Harbor Freight chain breaker, or a vise. These are cheap tools ($10-$15 depending on what's on sale) and are both easier and safer to use than a hammer.

I know many people have good luck with the Bike Smith tool. I'm not disagreeing with them. But many others do not.