Pretty bad, actually. There is no official documentation of the name mangling, so everything that is known is reverse engineered. In particular, there are outright bugs in the name mangling scheme, and one of the rules appears to rely on hashing the function body to produce a result. I'd give you examples, but, I don't have any of them on hand (I'm mostly relying on recollection from conversations with David Majnemer).

The Microsoft name mangling scheme is explained in Agner Fog's PDF on calling conventions: http://www.agner.org/optimize/calling_conventions.pdf

That's really only a fraction of it. The only definitive documentation that I know of is clang's implementation of its mangling scheme: https://github.com/llvm-mirror/clang/blob/master/lib/AST/Mic...