I'm just providing this as a fast alternative to the Semaphore, which as far as I know, will cause a kernel call every time.

Complicated is relative.  In terms of the condition variable api, I wouldn't say that it is.  But given the fact that we have to emulate condition variables on older windows, then yes, it is complex.

If we are rolling our own instead of using Semaphores (as has been suggested for performance reasons) then using a Condition variable is IMHO safer than a custom solution because the correctness of that approach is so easily provable.