They look pretty cool when decorated and used as embellishments. I've seen them used in jewelry making,too, but I've only used them for embellishments on LOs, tags, altered stuff, ATCs. Here are a couple examples:
I've used a couple when I had round embellishments of the perfect size (like a baseball sticker), and I bought a themed predecorated pack one time for a page (but that had everything to do with the theme and nothing to do with the bottlecaps), but mostly I am very literal about my embellishments, so my primary uses of them have been on a page about soda and a page about a sculpture made of... bottle caps.
I like bottle caps, but I haven't used them in LO's. The main reason I haven't is because I do not own a machine that I can smash them with. To smash them I would have to find another means. But I love looking at LO's that use them. And a "found item" that I'm wanting to incorporate into some of my LO's is soda can tabs.
I need some vintage Coke bottle caps. I mean the ones from the 1950's that acutally have cork in them. My dad worked for Coke then. I have seen them on line but haven't made a special effort to get them yet.
I made necklaces for my family members when my daughter went to the AF during basic training. I've used them as embellies on alot of my mini albums. You can smash them in the cuttlebug. They are just cute and vintage looking.