I have one, and I've used it a few times. It relies on the idea that the average color in a scene is neutral, so sometimes (when there is a dominant color) it doesn't work. When it does work, it gets you in the ballpark for both exposure and filter pack in about 1 sheet of paper (4 test exposures). The gray patch is sometimes a bit hard to interpret, and you usually need some final adjustments to the final filter pack for a given negative. But it does shorten the process for getting a basic filter pack for your setup. My procedure would be to use it to help determine a basic filter pack and then to make adjustments to the pack from full negative test exposures for each new negative. If you don't have an on easel analyzer, the cube is very handy for determining the exposure for a new negative, even if you make the filter adjustment based on a full print.