One thing I really wanted to be able to do with Propeller was a post-link optimization of a Go program. Go's compiler is both fast and terrible. If you glance at the text, opportunities for simple optimizations abound. Unfortunately the layout of the program, references between the debug information and the function entry points, and Propeller having been designed for C++ programs kept me from getting very far. I did get a little ways, but not far enough that I could keep a long-running Go program from crashing.