For me, Go has replaced C++ and Java for systems work. All of my projects at work for the past 6 months have been done in Go. I don't think we'll be going back.
But, really, good systems are based on well-defined protocols, which makes the language choice moot. (If you're passing Java or C++ objects over the wire, shame on you!)
I work on large distributed systems. I've found that Go is a much cleaner and simpler language than C++ or Java, by far. You get the compiled speed and control over memory layout of C++ with the garbage collection of Java. The concurrency model in Go is a natural fit for programs that make heavy use of disks and networks; it neatly sidesteps the threads vs events debate (and both of those can lead to some nasty programs). But most of all, the language is pleasant to use and the compilers are really fast (ever tried to compile a million line c++ or java source tree?).
But, really, good systems are based on well-defined protocols, which makes the language choice moot. (If you're passing Java or C++ objects over the wire, shame on you!)