I haven't used it myself but I've heard a lot of cocktail enthusiasts rating it very highly. Seems crazy to me but then I do ridiculous things for coffee, so I can't really judge :-)
Buy a largish tupperware container, say 2-5 gallons, and wrap the sides and top--not the bottom--in insulating foam. Fill with purified water and place in a conventional freezer for at least 12 hours for smaller containers, 24+ for larger ones.
The idea is that the bottom will chill faster while the air bubbles rise to concentrate the cloudiness at the top. You'll wind up with a nice block that's mostly clear, save for an inch or so at the top.
Tips:
* Remove all food products and put baking soda in the freezer. You don't want your ice to taste like fish sticks.
* When you cut the ice, leave it out at room temperature for ~10 minutes before cutting. Score lines in the block use a mallet and knife edge to separate pieces.
It makes it easier if you learn the timing such that it freezes to just about the depth you want to cut the blocks at (about 2") and the rest of the water is still liquid. Then the ice comes out already at melting point and cuts more easily.