I've primarily used them for layouts that were actually about soda or soda caps. But outside of that, they are also great if you have a 1-inch circle punch to punch out shapes or letters, which fit perfectly inside them. I used to flatten them using a sledge-hammer, but now I use my Sizzix Big Shot.
Here's one where I used them thematically (the page is about a sculpture made primarily of bottle caps):
Washington DC 2012 - Page 25 - Bottle Cap Giraffe. By Aquabunny
And I've used them several times for embellishments in page kits (in the first photo, the two items in the upper left photo; in the second photo, at the corner of the silver frame in the lower right):
Leap Year Kit for February Kit Swap - Embellishments 2 By Aquabunny
Baseball Kit for March Kit Swap By Aquabunny