Fell into a lava pit and died, and my only diamond pickaxe disappeared with me into the fiery depths of my obsidian farm, so I used it to get a new one, but as a general rule I don't use it because I like the actual mining just as much as the building. I'm also planning to use it to make a railway connecting two portals in the Netherworld to get me from one of my bases to another much quicker because finding enough iron ore to build it is fairly difficult. I know it's cheating, and I know it ruins some of the fun, but there wouldn't be anything left of the world if I had to mine all that iron.