I always always always use Excel.
Why?
You don't have as much of a learning curve, most people have used it and understand the concepts of opening, saving, etc.
Most companies have it. You don't have the "I can't open your file" aspect of things forcing you to constantly save your work, THEN save it again in a different format.
In the insert objects section, is pretty much everything you'd want as far as arrows, connectors, etc. It's not as easy to use as, say, Visio at first. And you lose the benefit of standardized formatting.
But in my years out there now, I have never had someone say "That's not the standard symbol for a ...." It's more speed and accuracy that they want.