I've been using btrbk with a local linux machine i use as a file server. Works well for incremental snapshot backups, no need to "unthaw" an image, I can directly fetch files from a previous snapshot. The only thing I haven't figured out with btrfs is how to efficiently handle incremental backs to S3. I guess there's not much choice than to use image diffs using btrfs-send because you don't have hard/ref links. But I don't like this because then if i want to retrieve a file from some version I'd have to have an extra 30 TB free to install the base image and progressively all the diffs up to the point I want to retrieve, seems a lot harder. So to make this reasonable I'd have to choose to make periodic non-incremental base images, starts getting complicated.