It's a pretty simple function, but it's like the Null custom function; it's main purpose is to clarify the code. A calculation becomes self-documenting when you can use...
It may be best to also create an IsWindows function. I would be happy with using not IsMac rather than creating another CustomFunction, but I think that's debatable. If only one of the two functions is to be created, I prefer IsMac simply because it's shorter and quicker to type.
- Dan Smith