I'm hoping not to have one at all. I know the date we conceived, so early dating is unnecessary. I don't want to find out the sex, so the "routine" ultrasound (which my insurance won't cover, anyway) is unnecessary as well.
If it becomes medically imperative, I of course will get one. When the benefit outweighs the risk, and the annoyance to the baby. Babies can feel/sense those sound waves from the us, and actually often try to move away from the source.
I know that Dopplers use essentially the same technology but at a lower scale/frequency, but it is for a shorter time, so I'm going to allow that. Plus, I REALLY want to hear the heartbeat!
Anyway, that's just my opinion. And I should add that DH is still wanting to find out if it's a boy or girl, so we'll see what comes of that!