This is one of the most dramatic sound-shaping tools I have ever encountered. I don't know if it is truly a great mastering tool.
I would expect that if you spent some serious time learning how to work with it, C3 might become a good mastering tool in the right hands. I have not spent more than a few hours attempting to master songs with it (because I hadto get something done for someone) , and then went back to using commercial toolkits: Waves C4, L3, Sony Oxford inflator, TC Master X5. When mastering, I tend to go for nuance and add some "sheen" like the cover of a glossy magazine, as well as boosting the level and fine tuning various aspects of the mix that I am working with.
The C3 is not quite as subtle as many commercial multiband compressors, such as the Waves C4, TC Master X3 and X5, and others. Subtle is usually what you want when mastering.
When doing sound design, having a tool that can create dramatic shifts in the timbre of whatever is going through it is very helpful.
Apulsoft's C3 is one of the more powerful and satisfying sound sculpting tools, and, it is free!