My house is going to need seven of these things (gulp) but I decided to go for it. The killer feature, if you're somebody who cooks and occasionally smokes up the kitchen, is that it will give you a "heads up" and you can walk over and wave your hand to silence it. No loud alarm.

The other great feature is that I'll never have to replace 9V batteries again. It seems that once every year or two, usually when the temperature first gets chilly in the fall, all the fire alarms in the house will start chirping at 2am. This is deeply annoying. With the new Nest, this problem goes away. (Presumably, there's a rechargeable lithium battery in there, and it charges from the 120V wires that my house already has for its original fire alarm.)

Interestingly, not only will you get notifications to your phone, but they'll also directly contact the fire department for emergencies. This suggests that a future feature might be a burglar alarm, since each smoke detector is also now a motion detector. The only missing piece of hardware to add would be some sort of cellular data path in case the landline is cut.