I never have the need to look down at my actual speedometer. It's so convenient having the HUD right there.
Now, if my Camaro didn't come with a HUD, I probably wouldn't install it afterwards, just because of the price. But then again, I have other priorities, and I don't have much money to spend by the end of the day anyways.
If you have the money, and you want a HUD, it's well worth the price.