Do not buy a Nest Protect. You will regret it.
You can stop or mute this video if it’s annoying, but you cannot stop a Nest.
Disclaimer: I am a Google employee. I paid for these myself. So I speak as myself.
UPDATE 2015-06-17: There is a new version of the Nest Protect hardware out now with an updated smoke sensor: The hardware shown in this video was the 1st generation hardware, not the new 2nd generation hardware.