Not a major user, but we're using zsync for system updates of our Raspberry Pi based digital signage operating system (https://info-beamer.com/hosted). It's pretty great and offers a few things we couldn't do with bittorrent: Every time we have a new release we put together an install.zip file of everything required (kernel, firmware files, initrd, squashfs). Users can download this file directly and unzip it on their SD card and it will boot our OS. For updates we use a previous (see below) version of our install.zip already available on the device and only download the changes. We then unzip that into a new partition for an A/B booting.

Zsync is awesome as we can specify any number of existing files already available on the device (with the -i command line option) and zsync will try to make use of them to minimize downloads. We really use this feature to our advantage: zsync by default will keep the previous version of a file if it's going to overwrite it. So we have two versions of install.zip on a device. When switching between OS releases (stable / testing...) we can switch back and forth with zero additional downloads as both versions are available and zsync makes use of that. Similarly after a user installs our OS, we just have the unpacked artefacts (kernel, etc.) on the SD. We can quickly recreate an initial version of the install.zip file on the device by seeding the download with those files. It's usually just 500k to construct an initial install.zip file we then later use to minimize all future updates.