I purchased a couple of units so change out my existing smoke alarms and all units do what they promise to do after a little effort to get them working.
One unit was giving a low battery alarm which was fixed by taking the batteries out and putting them in again (as in the other review). One unit was giving a low battery alarm which could only be fixed by replacing the batteries (which come with a 5 year guarantee) with fresh ones, Amazons answer was to offer to replace the whole unit but with some negotiations they offered to refund the cost of new batteries instead.
Setting up the units was easy enough once I mastered to switch the Wi-Fi networks between the internet and Nest units in time during the installation process and ensured that all units are not too far away from each other. Be prepared to swap into the Wi-Fi settings of your device very quickly as otherwise the installation might fail and you need to repeat it from the beginning.
I like the pathlight during the night as I have the units in the hallway between bedroom and restroom and it saves me finding the light switch as it is bright enough to guide me through the hallway.
Overall a good investment, they might be more expensive than a standard smoke detector but the combined functionality with the added benefit that all units will alert you in case of an incident make me fell safer.
Too bad the units are already sold out 4 days after being available first.