I have ISO's in a private company and am trying to decide whether there is any advantage in exercising them. The price is low enough and my confidence in the company is high enough that I am comfortable taking a risk to exercise them.

I understand the tax implications with an exercise with respect to AMT with a public company as the market value is easy to determine.

I do not understand the same implications with respect to a privately held company that will one day be acquired.

Does anyone know reasons to start exercising now or reasons to hold off until an acquisition?