We are using it to serve content for our image-heavy special marketing campaign pages and mobile apps, heavily using it to serve different image sizes based on device (we are mobile first and have to look good on all devices). We have always been bound by bandwidth, never by any other metric (~1TB / month). In addition, their automatic conversion (our designers for example sometimes upload .ai files) and image minimization helps us to speedup delivery & minimize bandwidth on the one side, on the other side it makes live easier for our designers and more flexible for us if we later want to use the same image somewhere else.
Last, but not least, their image manipulation with overlays allows very fancy stuff which we use to generate "sticker" images (see the URLs here: api-md15.dealini.ch/cardUnits) and we could not replicate this feature with any of their competition.
One final note: Their engineering team is super-fast, e.g. it took them ~1 week from "we need to use .ai files as transparent overlays" to implementation and deployment. And this happened for several feature requests!