The smoke detector goes off all the time -- without rhyme or reason. With a normal detector, you can just pull the battery if it refuses to shut off. With Nest, you actually need a screw driver to open the battery compartment to turn it off.
So, when it decides to wake up the entire house at 3am (yes, the entire house, since they all go off in unison) you'll need to go find a screw driver to get it to shut up.
The one flippin' reason I bought a Nest Protect was so I could "wave to silence" an alarm. Well, every time it goes off, it tells me that "this alarm cannot be silenced" - even by pushing the power button. Not once have I been able to turn off an alarm without removing the batteries.
The worst feeling is when you get an alert on your iPhone while nobody (but your pet) is home. "Alert: your house is on fire." Then you rush home and realize it's a false alarm.
After our fourth or fifth false alarm we returned them all and replaced them with the old-fashioned alarms.
Watch this hilarious video if you're considering buying a Nest Protect: https://www.youtube.com/watch?v=BpsMkLaEiOY (warning: loud alarms in video)