And if you leave the company they actually cost you money, since you have to pay out of pocket to exercise them (for stock that may end up never being sellable) and you also have to pay AMT taxes based on the latest company valuation.
http://www.startupcompanylawyer.com/2009/01/11/should-a-comp...
If you exercise and sell at the same time, you will pay short-term income taxes, but without any AMT to worry about.
I agree that exercising and selling can be a good strategy but we're (mostly) talking about private companies here where that may not be an option due to a lack of liquidity.