intuitive, attention to detail: excellent!

Ordered to its output in Europe (from the UK, only available at the time), it is installed now for almost 2 years, in the entrance of the apartment (ceiling so between living room, kitchen and hallway to rooms).
Suffice to say that such a long period, we were able to get concrete advice and definitively conquered.

It has triggered rare times to smoke only when hazardous firings. Very effective point spoken Early Warning (such as AI in science fiction movies, audible, not robotics to a sub) and at the same time notifications on iOS and iPad (wherever you may be).
A point used daily, the pilot, very sweet when we pass beneath the night with a pulsating effect, without rushing.
An exemplary battery life (original battery never changed in 2 years, pilot power mode "average" active).
Regular updates of the application (iOS), very intuitive and efficient, besides being elegant.
Not really mind about the carbon monoxide detection (a priori unnecessary in our habitat) but LEDs green in the app reassure on its discrete and continuous presence.
Note the simple installation, he just had to find adapted to our ankles ceiling (percussion drill and saw the concrete slab). My wireless network was masked (need the name and password to access), the connection was erratic; so I solved everything by making the network visible (and of course still protected by password).

Note on Customer Service: there was the launch function "wave" where you could disable alerts with a simple arm movement under the detector. For safety standards reasons, this function has been disabled (I understand that the detector virtually risked disengage even when danger). Nest had the courage of a product recall, always bad in terms of image and immediately offered me a refund (refused for one, saw the product quality beyond this gadget optional).
Very professional and efficient, Bravo!

The budget has nothing to do with a conventional detector indeed. But the ease of use (update, invisible product except when it is needed), the quality and elegance of all the attention to detail in the app, warrant.