What is its level of maturity on non x86 platforms such as ARM? It might be a nice alternative to C and especially C++ on small embedded boards, but then any smaller overhead would matter a lot more than on a PC.
Fully operational. On arm/linux it is trivially easy to use, tends to just work out of the box (ldc offers packaged downloads on their website, gdc is in the Raspbian repository, etc). If you are doing bare metal and/or strictly limited hardware resources it is a little more work to be productive with minimal runtime but still not hard if you're familiar with how to do it.