MARS' tag-line at the 1&1 web hosting company is "replicating petabytes over long distances" and "has replaced DRBD as the backbone of the 1&1 geo-redundancy feature as publicly advertised for 1&1 Shared Hosting Linux (ShaHoLin). MARS is also running on several other 1&1 clusters. Some other people over the world have also seemingly started to use it."
MARS in 2016 saw several bug fixes, portability improvements for newer kernels, some developments around new features, and more. Thomas Schoebel-Theuer who has been leading MARS development at 1&1 is hoping to see more upstream developers willing to engage in the project and ideally work to see it upstreamed. It looks like there's still more work to do, but maybe we'll see this giant code-base land in the Linux kernel in 2017.
Those interested in MARS for storage replication can find out more via the annual status update, this 2016 PDF presentation on the project, or this GitHub repository.