It's used heavily in application code for automotive ECUs. It has code generators that generate output with a really, really, small footprint and zero runtime overhead; due to optimization at very high levels, this is on the level of really good LTO optimization.

Drawbacks: it's very un-agile; you really have to think the system through completely. (The magic being that if you do this, it is very likely correct by design.) It's not really feasible to specify a part of the system now and leave other parts open for later refinement. The other drawback being that no good non-commercial options exist.