Word Clouds are a great way of visualising the relative 'size' of different categories of your data. While they might not be effective for making precise comparisons, they can definitely be useful for providing a high impact visualisation that people find more striking and memorable than other ways of visualising the same data.
For example Here we have the sales by sub-category as a bar chart:
And here we have the same view as a word cloud.
And while you might not be able to give a precise value for the word cloud you can intuitively see, which are the larger sub categories are, and which are the smaller.
Creating a word cloud is a really simple process, simply place a String dimensions on the text portion of the marks card, a measure on the size portion of the card, and then change the type of mark to text and voila! a word cloud!