Monte Carlo simulation of x-ray instrumentation has been used for many years — in particular the SHADOW package [WCC94; Rio+11] has found widespread use, and is still actively developed, yet has some limitations. Ray[Sch08] and Xtrace[Bau+07] are other packages using the same basic techniques.
What sets McXtrace apart is, among other things, its open source development strategy and its inherent modularity. This combination lets independent scientists work indepently on separate modules that they have use for, and contribute to the whole with only a small effort.